drm/amdgpu: SI support for UVD and VCE power managment

Port functionality from the Radeon driver to support
UVD and VCE power management.

Signed-off-by: Alex Jivin <alex.jivin@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>