drm/sun4i: frontend: Add helpers for input data mode and pixel sequence

This introduces new helpers for retrieving the input data mode and pixel
sequence register field values based on the DRM format instead of
hardcoding these. This makes it easier to add support for more formats.

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/20181123092515.2511-18-paul.kocialkowski@bootlin.com