drm/sun4i: backend: Check for the number of alpha planes

Due to the way the composition is done in hardware, we can only have a
single alpha-enabled plane active at a time, placed in the second (highest
priority) pipe.

Make sure of that in our atomic_check to not end up in an impossible
scenario.

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/7371f62a1385f2cbe3ed75dfca2e746338eb2286.1516617243.git-series.maxime.ripard@free-electrons.com