[Why]
vblank_workqueue is never released.

[How]
Free it upon dm finish.
Tested-by: NDaniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: NQingqing Zhuo <qingqing.zhuo@amd.com>
Reviewed-by: NNicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: NSolomon Chiu <solomon.chiu@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>