drm/amd/display: update pixel format in DP hw sequence

[WHY]
DP 420 formats do not light up because the pixel processing mode
of the DP_FORMAT is misprogrammed

[HOW]
Added appropriate programming for DP pixel format

Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Yihan Zhu <yihan.zhu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>