drm/sun4i: Pass pointer for underlying backend into layer init

sun4i_layer only controls the backend hardware block of the display
pipeline.

Pass pointers to the underlying backend in the layer init function,
instead of trying to fetch it from the drm_device structure. This
avoids the headache of trying to figure out which device the layers
actually belong to.

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