drm/amdgpu: enable mmhub clock gating for Arcturus

Init MC_MGCG/LS flag. Also apply to athub CG.

Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Kevin Wang <kevin1.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>