drm/amd/display: unit show garbage when do OPTC blank

[Why]
Unit enter to S4, garbage show on screen when do OPTC blank.

[How]
Wait for vblank then do OPTC blank

Signed-off-by: Paul Hsieh <paul.hsieh@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>