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.