drm/amdgpu: enable uvd dpm and powergating

Enable UVD dpm (dynamic power management) and powergating.  UVD dpm dynamically scales the UVD
clocks on demand.  Powergating turns off the power to the block when it's not in use.

Signed-off-by: Sonny Jiang <sonny.jiang@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>