drm/sun4i: frontend: Replace ARGB with XRGB as supported format

The frontend documentation (for the A33) mentions that ARGB is supported
as output, but with the alpha component always set to 0xff. In practice,
this means that the alpha component cannot be preserved when going
through the frontend. Since the information is lost, ARGB is not
properly supported.

As a result, expose the matching format supported by the frontend (both
for input and output) as XRGB instead of ARGB.

Since ARGB was the selected format for connecting the frontend to the
backend, change it to XRGB to reflect this as well.

The A31 and A80 SoCs apparently have a bit to enable proper alpha,
but this is not supported at this point (see the comment already in the
code).

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/20181123092515.2511-3-paul.kocialkowski@bootlin.com