[Why]
Need separate feature flag for DP 4:2:0 support, since existing
flag is used for HDMI

[How]
Added dp_ycbcr420_supported to struct encoder_feature_support
Signed-off-by: NEric Bernstein <eric.bernstein@amd.com>
Reviewed-by: NTony Cheng <Tony.Cheng@amd.com>
Acked-by: NBhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>