drm/sun4i: sun4i: Introduce a quirk for lowest plane alpha support

Not all sunxi platforms with the first version of the Display Engine
support an alpha component on the plane with the lowest z position
(as in: lowest z-pos), that gets blended with the background color.

In particular, the A13 is known to have this limitation. However, it was
recently discovered that the A20 and A33 are capable of having alpha on
their lowest plane.

Thus, this introduces a specific quirk to indicate such support,
per-platform. Since this was not tested on sun4i and sun6i platforms, a
conservative approach is kept and this feature is not supported.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180719080838.31598-2-paul.kocialkowski@bootlin.com