drm/sun4i: backend: Add a helper and a list for supported formats

In order to check whether the backend supports a specific format, an
explicit list and a related helper are introduced.

The prototype of this helper is added to the header so that it can be
called from sun4i_layer later (when introducing tiled mode support).

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/20181123092515.2511-5-paul.kocialkowski@bootlin.com