drm/sun4i: Move layers from sun4i_drv to sun4i_crtc

This patch moves the sun4i_layers_init call from sun4i_drv_bind to
sun4i_crtc_init, and the layers pointer from struct sun4i_drv to
struct sun4i_crtc.

The layers are bound to a specific crtc, and they are not directly
used once initiated. They are used through their included drm_plane
structures.

Moving the layers into the crtc facilitates binding them to the crtc
explicitly, by setting the corresponding bit in their .possible_crtcs
fields right after the crtc is initialized. This is done in a later
patch.

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