drm: Add data transmission order bus flag

The flags indicate whether data is transmitted LSB to MSB or MSB to LSB
on the bus.

The exact meaning is bus-type dependent. For instance, for LVDS buses
the flags indicate whether the seven data bits transmitted in a clock
pulse are sent in normal order (MSB to LSB, slots 0 to 6) or reverse
order (LSB to MSB, slots 6 to 0).

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>