drm/amdgpu:All UVD instances share one idle_work handle

All UVD instanses have only one dpm control, so it is better
to share one idle_work handle.

Signed-off-by: James Zhu <James.Zhu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Tested-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>