drm: mxsfb: Use bus_format from the nearest bridge if present

In case there is a bridge connected to the LCDIF, use bus_format
from the bridge, otherwise behave as before and use bus_format
from the connector. This way, even if there are multiple bridges
in the display pipeline, the LCDIF will use the correct format.

Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Daniel Abrecht <public@danielabrecht.ch>
Cc: Emil Velikov <emil.l.velikov@gmail.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Stefan Agner <stefan@agner.ch>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210726194457.341696-1-marex@denx.de