drm/sun4i: backend: Disable alpha on the lowest plane on the A20

Unlike we previously thought, the per-pixel alpha is just as broken on the
A20 as it is on the A10. Remove the quirk that says we can use it.

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/20200728134810.883457-2-maxime@cerno.tech