Comment # 8 on bug 1027378 from Stefan Dirsch

Wel�� we don't want to prevent our user to try also amdgpu driver. But default
is radeon and not amdgpu. amdgpu will also be loaded but won't be in use. So
radeon X driver is loaded, not amdgpu driver. If you want amdgpu you need to
blacklist radeon as you apparently did.

I recommend to use radeon driver for these devices. Fixing issues in SI/CIK
support for amdgpu KMS driver will be low priority for obvious reasons.
Interesting in bisecting the kernel?