drm/amd/display: DP YCbCr 4:2:0 support

Update MSA MISC1 bit 6 programming to handle YCbCr 4:2:0
and BT2020 cases.

Signed-off-by: Eric Bernstein <eric.bernstein@amd.com>
Reviewed-by: Hersen Wu <hersenxs.wu@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>