[Why] BT2020 is not supported in COLOR_ENCODING property of planes. Only BT601 and BT709 was available. [How] Allow BT2020 as legit value in setting COLOR_ENCODING property. Signed-off-by: Stylon Wang <stylon.wang@amd.com> Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com> Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com> Signed-off-by: Alex Deucher <alexander.deucher@amd.com>