drm/sun4i: backend: Support YUV planes

Now that we have the guarantee that we will have only a single YUV plane,
actually support them. The way it works is not really straightforward,
since we first need to enable the YUV mode in the plane that we want to
setup, and then we have a few registers to setup the YUV buffer and
parameters.

We also need to setup the color correction to actually have something
displayed.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/66088c1398bd3189123f28a89a7ccc669fe9f296.1519931807.git-series.maxime.ripard@bootlin.com