How to diagnose/stop gdm-x-session spamming syslog

[Note: this is similar to issue #418 in the gnome gitlab but it happens to me constantly and I'm not running Evolution. I filed this there too, but gnome 3.28.2 is no longer supported there.]

I'm not sure how long this has been going on or what triggered it, but I recently noticed that my syslog is being spammed with the same 3 lines repeatedly. For example:

$ tail -f /var/log/syslog
Jul 10 04:35:53 496-MBP /usr/lib/gdm3/gdm-x-session[2094]: (II) RADEON(0): EDID vendor "APP", prod id 41006
Jul 10 04:35:53 496-MBP /usr/lib/gdm3/gdm-x-session[2094]: (II) RADEON(0): Printing DDC gathered Modelines:
Jul 10 04:35:53 496-MBP /usr/lib/gdm3/gdm-x-session[2094]: (II) RADEON(0): Modeline "2880x1800"x0.0  337.75  2880 2928 2960 3040  1800 1803 1809 1852 +hsync -vsync (111.1 kHz eP)
Jul 10 04:35:53 496-MBP /usr/lib/gdm3/gdm-x-session[2094]: (II) RADEON(0): EDID vendor "APP", prod id 41006
Jul 10 04:35:53 496-MBP /usr/lib/gdm3/gdm-x-session[2094]: (II) RADEON(0): Printing DDC gathered Modelines:
Jul 10 04:35:53 496-MBP /usr/lib/gdm3/gdm-x-session[2094]: (II) RADEON(0): Modeline "2880x1800"x0.0  337.75  2880 2928 2960 3040  1800 1803 1809 1852 +hsync -vsync (111.1 kHz eP)
Jul 10 04:35:53 496-MBP /usr/lib/gdm3/gdm-x-session[2094]: (II) RADEON(0): EDID vendor "APP", prod id 41006
Jul 10 04:35:53 496-MBP /usr/lib/gdm3/gdm-x-session[2094]: (II) RADEON(0): Printing DDC gathered Modelines:
Jul 10 04:35:53 496-MBP /usr/lib/gdm3/gdm-x-session[2094]: (II) RADEON(0): Modeline "2880x1800"x0.0  337.75  2880 2928 2960 3040  1800 1803 1809 1852 +hsync -vsync (111.1 kHz eP)
Jul 10 04:35:53 496-MBP /usr/lib/gdm3/gdm-x-session[2094]: (II) RADEON(0): EDID vendor "APP", prod id 41006
Jul 10 04:35:53 496-MBP /usr/lib/gdm3/gdm-x-session[2094]: (II) RADEON(0): Printing DDC gathered Modelines:
Jul 10 04:35:53 496-MBP /usr/lib/gdm3/gdm-x-session[2094]: (II) RADEON(0): Modeline "2880x1800"x0.0  337.75  2880 2928 2960 3040  1800 1803 1809 1852 +hsync -vsync (111.1 kHz eP)

etc. (at least 8 times per second)

I'm trying to figure out (and fix) what's causing this. I'd appreciate any tips/pointers as to how to diagnose this.

I'm running Ubuntu on an older (2015-ish) MacBook Pro with Gnome 3.28.2 (and all apt-installed sw up-to-date):

$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 18.04.4 LTS
Release:    18.04
Codename:   bionic

$ apt-cache policy gdm3
gdm3:
  Installed: 3.28.3-0ubuntu18.04.4
  Candidate: 3.28.3-0ubuntu18.04.4
  Version table:
 *** 3.28.3-0ubuntu18.04.4 500
        500 http://ca.archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages
        500 http://security.ubuntu.com/ubuntu bionic-security/main amd64 Packages
        100 /var/lib/dpkg/status
     3.28.0-0ubuntu1 500
        500 http://ca.archive.ubuntu.com/ubuntu bionic/main amd64 Packages

$ apt-cache policy xserver-xorg
xserver-xorg:
  Installed: (none)
  Candidate: 1:7.7+19ubuntu7.1
  Version table:
     1:7.7+19ubuntu7.1 500
        500 http://ca.archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages
     1:7.7+19ubuntu7 500
        500 http://ca.archive.ubuntu.com/ubuntu bionic/main amd64 Packages

$ uname -a
Linux 496-MBP 5.3.0-62-generic #56~18.04.1-Ubuntu SMP Wed Jun 24 16:17:03 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

$ inxi
CPU~Quad core Intel Core i7-4980HQ (-MT-MCP-) speed/max~798/4000 MHz Kernel~5.3.0-62-generic x86_64 Up~51 min Mem~2430.2/15889.5MB HDD~1000.6GB(35.4% used) Procs~337 Client~Shell inxi~2.3.56

$ xrandr
Screen 0: minimum 320 x 200, current 2880 x 1800, maximum 16384 x 16384
eDP connected primary 2880x1800+0+0 (normal left inverted right x axis y axis) 331mm x 207mm
   2880x1800     59.99*+
   1920x1200     59.95  
   1920x1080     60.00  
   1600x1200     59.95  
   1680x1050     60.00  
   1400x1050     60.00  
   1280x1024     59.95  
   1440x900      59.99  
   1280x960      59.99  
   1280x854      59.95  
   1280x800      59.96  
   1280x720      59.97  
   1152x768      59.95  
   1024x768      59.95  
   800x600       59.96  
   848x480       59.94  
   720x480       59.94  
   640x480       59.94  
DisplayPort-0 disconnected (normal left inverted right x axis y axis)
DisplayPort-1 disconnected (normal left inverted right x axis y axis)
HDMI-0 disconnected (normal left inverted right x axis y axis)

$ lspci -nn | grep -E 'VGA|Display'
01:00.0 VGA compatible controller [0300]: Advanced Micro Devices, Inc. [AMD/ATI] Venus XT [Radeon HD 8870M / R9 M270X/M370X] [1002:6821] (rev 83)

Please let me know if there's other information that could help with a diagnosis.

Thanks!