uterm: video_fbdev: discard non-xrgb32 devices

uterm: video_fbdev: discard non-xrgb32 devices

We discard all non-xrgb32 devices for simplicity reasons for now. We
definitely need to support other devices in future but that makes the
backend unnecessarily complex at the beginning so avoid it.

This also moves the resolution-change to the beginning and duplicates the
tests so we can be totally sure that everything is right after setting the
correct values.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>