drm/sun4i: backend: Remove ARGB spoofing

We've had some code for quite some time to prevent the alpha bug from
happening on the lowest primary plane. Since we now check for this in our
atomic_check, we can simply remove it.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/965180afd8169ccaa848f061d16a2c1a9ec4d289.1518802627.git-series.maxime.ripard@bootlin.com