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

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

Just like in the backend, the prototype of the helper is added to the
frontend header so that it can be used later on. The helper is also
exported because it will be used outside of the frontend module.

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/20181123092515.2511-6-paul.kocialkowski@bootlin.com