drm/amd/display: clean up encoding checks

[Why]
All ASICS we support has YCbCr support, so
the check is unnecessary, the currently logic
in validate output also returns true all
the time, so the unneccessary logic is removed

Signed-off-by: Eric Yang <Eric.Yang2@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>