drm/amdgpu/radeon: Use radeon by default for CIK GPUs

Even if CONFIG_DRM_AMDGPU_CIK is enabled.

There is no feature parity yet for CIK, in particular amdgpu doesn't
support HDMI/DisplayPort audio without DC.

v2:
* Clarify the lack of feature parity being related to HDMI/DP audio.
* Fix "SI" typo in DRM_AMDGPU_CIK help entry.

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>