drm/sun4i: Fix formats usable by the primary plane

Even though all our planes can support the ARGB formats, the lowest plane
(ie the primary plane) cannot use the alpha component, otherwise it will
just result in the transparent area being entirely black.

Since some applications will still require the ARGB format, let's force the
format to XRGB to drop the alpha component entirely.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>