drm/amdgpu/vcn2: don't access register when power gated

It will cause bus hang to access register UVD_STATUS
when VCN is in the state of power gated.

Signed-off-by: Jack Xiao <Jack.Xiao@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>