drm/amd/display: mpcc black color should not be impacted by pixel encoding format

[Why]
The format in MPCC should be 444

[How]
do not modify the mpcc black color according to pixel encoding format

Signed-off-by: Xiaodong Yan <Xiaodong.Yan@amd.com>
Reviewed-by: Eric Yang <eric.yang2@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>