drm/amd/display: fix DP 422 VID_M half the rate issue.

drm/amd/display: fix DP 422 VID_M half the rate issue.

[Description]
when programming VID_TIMING, we were using the original VESA timing for DP_VIDM/N.
for YCbCr420 or compressed YCbCr422, using half rate as  YCbCr444.

Signed-off-by: Charlene Liu <charlene.liu@amd.com>
Reviewed-by: Nikola Cornij <Nikola.Cornij@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>