drm/amdgpu/jpeg3: add cancel_delayed_work_sync before power gate

Add cancel_delayed_work_sync before set power gating state
to avoid race condition issue when power gating.

Signed-off-by: James Zhu <James.Zhu@amd.com>
Reviewed-by: Leo Liu <leo.liu@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org