As we are going to add support for the Allwinner DE2 Mixer in sun4i-drm
driver, we will finally have two types of layers.

Each layer is bound to a drm_plane that is CRTC-specific, so we create
them when initializing CRTC (calling sun4i_layers_init, which will be
generalized in next patch). The drm_plane's will be used when creating
CRTC, but the CRTC initialization code do not care other properties of
the layer, so we let the sun4i_layers_init function return drm_plane's
only.

As we have no need to trace the layers after the CRTC is properly
created, we drop the layers pointer in sun4i_crtc struct.

Doing this uncouples the CRTC code from the type of layer (the
sun4i_layers_init function name is still hardcoded and will be changed
in the next patch), so that we can finally gain support for the
mixer in DE2, which has different layers.
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>