drm/amdgpu: split athub clock gating from mmhub

Untie the bind of get/set athub CG state from mmhub, for cosmetic fix and Asic
not using mmhub 1.0. Besides, also fix wrong athub CG state in amdgpu_pm_info.

Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>