drm/sun4i: backend: Wire in the frontend

Now that we have a driver, we can make use of it. This is done by
adding a flag to our custom plane state that will trigger whether we should
use the frontend on that particular plane or not.

The rest is just plumbing to set up the backend to not perform the DMA but
receive its data from the frontend.

Note that we're still not making any use of the frontend itself, as no one
is setting the flag yet.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/cdffc25eab2d817820cc78cbd24f1f4b99902014.1516613040.git-series.maxime.ripard@free-electrons.com

drivers/gpu/drm/sun4i/sun4i_layer.h

@@ -24,6 +24,7 @@ struct sun4i_layer {
 
 struct sun4i_layer_state {
 	struct drm_plane_state	state;
+	bool			uses_frontend;
 };
 
 static inline struct sun4i_layer *