Comment # 206 on bug 91880 from Sandeep

You've got to make sure that radeon and amdgpu drivers don't clash.

Try the following kernel parameters: radeon.si_support=0 radeon.cik_support=0
amdgpu.si_support=1 amdgpu.cik_support=1 amdgpu.dpm=1

That's what I use and it works fine for me.