drm/amdgpu: Add runtime VCN PG support

Enable support for dynamically powering up/down VCN on demand.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>