drm/sun4i: Fix DE2 VI layer format support

DE2 VI layer doesn't support blending which means alpha channel is
ignored. Replace all formats with alpha with "don't care" (X) channel.

Fixes: 7480ba4d

 ("drm/sun4i: Add support for DE2 VI planes")
Acked-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20200224173901.174016-4-jernej.skrabec@siol.net