drm/sun4i: backend: Refine the logic behind using the frontend

Checking that scaling is in use is not sufficient as a condition to
decide to use the frontend.

Since not all layer formats are supported by the frontend, we need to
check for that support first. Then, the frontend must only be enabled
if the backend doesn't support the format or that scaling is required.

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/20181123092515.2511-7-paul.kocialkowski@bootlin.com