Comment # 1 on bug 101478 from Michel Dänzer

[    3.750697] amdgpu 0000:01:00.0: SI support provided by radeon.
[    3.750698] amdgpu 0000:01:00.0: Use radeon.si_support=0 amdgpu.si_support=1
to override.

You'll need to follow this suggestion.