drm/sun4i: frontend: Add proper definitions for format registers

This introduces proper definitions for the input and output format
configuration registers instead of a macro and raw values in the code,
with the intent to increase code readability and reduce indirections.

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/20181123092515.2511-19-paul.kocialkowski@bootlin.com