drm/amd/display: Expose DRM_FORMAT_RGB565 on overlay planes

RGB565 support isn't restricted to just the primary plane in DC, so
also expose support for it on overlays.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: David Francis <david.francis@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>