drm/amdgpu/jpeg: Hold pg_lock before jpeg poweroff

Acquire jpeg_pg_lock before changes to jpeg power state
and release it after power off from 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>