drm/sun4i: backend: Support alpha property on lowest plane

Unlike what we previously thought, only the per-pixel alpha is broken on
the lowest plane and the per-plane alpha isn't. Remove the check on the
alpha property being set on the lowest plane to reject a mode.

Fixes: dcf496a6

 ("drm/sun4i: sun4i: Introduce a quirk for lowest plane alpha support")
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Cc: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200728134810.883457-1-maxime@cerno.tech