(In reply to John Frei from comment #36)
> I assume that the second try (boot with unmodified kernel parameters) the
> module radeon was used.
> The only thing I can try is to compile the kernel without the 'amdgpu CIK
> parts', but this should not affect the radeon module, right?

Either disable CIK support in amdgpu or blacklist the amdgpu module (e.g.,
modprobe.blacklist=amdgpu on the kernel command line in grub) to keep the
amdgpu driver from loading on CIK hardware.