drm/sun4i: Drop hardcoded .possible_crtcs values from layers

To support multiple display pipelines, we would have multiple crtcs,
with one or more planes bound to them. Obviously having hardcoded
values for the drm_plane .possible_crtcs field is not going to work.

For primary and cursor planes, the value is set by
drm_crtc_init_with_planes. We just need to set it for overlay planes.

We also fix the value set for the RGB encoder, by referencing the
crtc set in sun4i_drv.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>