drm/amdgpu/vcn: Hold pg_lock before vcn power off

Acquire vcn_pg_lock before changes to vcn power state
and release it after power off in idle work handler.

Signed-off-by: Sathishkumar S <sathishkumar.sundararaju@amd.com>
Reviewed-by: Leo Liu <leo.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>