drm/sun4i: backend: Use a specific function to check if a plane is supported

Before this patch, it is assumed that a plane is supported either
through the frontend or through the backend alone. However, the DRM
interface does not allow finely reporting our hardware capabilities
and there are cases where neither are support.

In particular, some plane formats are supported by the backend and not
the frontend, so they can only be supported without scaling.

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/20181123092515.2511-8-paul.kocialkowski@bootlin.com