drm/amdgpu: enable gfx clock gatings for navi12

enables following gfx clock gating features:

- medium grained clock gating
- medium grained light sleep
- coarse grained clock gating
- cp memory light sleep
- rlc memory light sleep

CGLS (Coarse Grained Light Sleep) will break s3, so don't enable it.

Signed-off-by: Xiaojie Yuan <xiaojie.yuan@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>