I have tried several attempts but without success. AMDGPU-PRO 17.40, 17.50, ROCm, Oibaf PPA, Kernel 4.16 and something more, but nothing makes OpenCL working with the AMD card and clinfo never gives right results.
Can I only wait the next AMDGPU-PRO driver compatible with Ubuntu 18.04 or could I do something different?
You can try to install their experimental driver from here:
https://support.amd.com/en-us/kb-articles/Pages/Radeon-Software-for-Linux-18.20-Early-Preview-Release-Notes.aspx
Hope it works for you
I'm in the same boat - for completeness here is the thread I started. @Mods, please consolidate if it makes sense to do so.
Intel/AMD Hybrid graphics Ubuntu 18.04