drm/sun4i: backend: Remove BGRX8888 from list of supported formats

The display backend does not support BGRX8888. There is also no trace
of this in the original list of supported formats before the commit
b636d3f9 ("drm/sun4i: frontend: Add support for the BGRX8888 input
format"). Nor do the backend configuration helpers handle this format.

Remove BGRX8888 from list of supported formats by the backend.

Fixes: 3d4265f8 ("drm/sun4i: backend: Add a helper and a list for supported formats")
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190125032314.20915-4-wens@csie.org