drm/sun4i: frontend: Determine input mode based on the number of planes

Use the number of planes associated with the DRM format to determine the
input mode configuration instead of the format iteself. This way, the
helper can be used for all packed formats without future changes.

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/20181123092515.2511-20-paul.kocialkowski@bootlin.com