drm/amd/display: Remove 4:2:2 DSC support

[why]
If DSC is available, a higher picture quality is achieved by using
DSC with 4:4:4 format. Using 4:2:2 instead does not offer any benefit
and would only introduce loss of quality. Removing it reduces
maintenance and testing effort.

Signed-off-by: Nikola Cornij <nikola.cornij@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>