drm/amd/display: Enable BT2020 in COLOR_ENCODING property

[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>