[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: NStylon Wang <stylon.wang@amd.com>
Reviewed-by: NNicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: NRodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>