drm/amd/display: The external monitor will show gray screen during SUT reboot

[Why]
same with CL#1711022(correcting yuv420 black color in function dcn10_blank_pixel_data,program_scaler),
yuv420 black color also needs to be correct when enabling HDMI stream at the resume procedure.

[How]
correcting the yuv420 black color according to the way how 420 is packed :2 channels carry Y component,
1 channel alternate between Cb and Cr.

Signed-off-by: yanyan kang <Yanyan.Kang@amd.com>
Reviewed-by: Eric Yang <eric.yang2@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>