drm/ingenic: Add support for serial 8-bit delta-RGB panels

Add support for 24-bit panels that are connected through a 8-bit bus and
use delta-RGB, which means a RGB pixel ordering on odd lines, and a GBR
pixel ordering on even lines.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201119155559.14112-4-paul@crapouillou.net