drm/sun4i: Add backend pointer to sun4i_layer

sun4i_layer only controls the backend hardware block of the display
pipeline. Instead of getting a pointer to the underlying backend
through the drm_device structure, leave one in itself.

Also drop the drm_device pointer, since it is no longer needed.

The next step forward would be to pass the pointer in through
sun4i_layers_init as a parameter. This would make it easier to support
multiple display pipelines layer on.

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