drm/sun4i: backend: Check that we only have a single YUV plane

Just like for the frontend, a single plane can use a YUV format. Make sure
we have that constraint covered in our atomic_check.

This is preliminary to the actual YUV support to make sure we don't end up
in an impossible to support situation.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/2f8586493d9139b12efe7e94f65e9a149f818e0e.1519931807.git-series.maxime.ripard@bootlin.com