The bus format both specifies the bpc and the way the individual bits get
serialized into the 7 LVDS timeslots.

While the is only one standard mapping for 6 bpc and so the driver could
infer the bit mapping from the bpc alone, there are more options for the
8 bpc case which makes specifiying the bus format mandatory.
To keep things consistent across panels and to set a precedent for new
panel additions add the proper bus format.
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Thierry Reding <treding@nvidia.com>