drm/amd/display: fix YCbCr420 blank color

drm/amd/display: fix YCbCr420 blank color

[ Upstream commit 12750d1647f118496f1da727146f255f5e44d500 ]

[Why]
YCbCr420 packing format uses two chanels for luma, and 1
channel for both chroma component. Our previous implementation
did not account for this and results in every other pixel having
very high luma value, showing greyish color instead of black.

YCbCr444 = <Y1, Cb1, Cr1>; <Y2, Cb2, Cr2> .....
YCbCr420 = <Y1, Y2,  Cb1>; <Y3, Y4,  Cr1> .....

[How]
Program the second channel with the black color value for luma
as well.

Signed-off-by: Eric Yang <Eric.Yang2@amd.com>
Reviewed-by: Hugo Hu <Hugo.Hu@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>