drm/sun4i: Remove the plane description structure

The plane description structure was mostly needed to differentiate the
formats usable on the primary plane (because of its lowest position), and
assign the pipes. Now that both are dynamically checked and assigned, we
can remove the static definition.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/6b09e3698e692c3338f70a5ae1e5a580f9dd08ee.1518802627.git-series.maxime.ripard@bootlin.com