drm: mxsfb: Set fallback bus format when the bridge doesn't provide one

If a bridge doesn't do any bus format handling MEDIA_BUS_FMT_FIXED is
returned. Fallback to a reasonable default (MEDIA_BUS_FMT_RGB888_1X24) in
that case.

This unbreaks e.g. using mxsfb with the nwl bridge and mipi dsi panels.

Reported-by: Martin Kepplinger <martink@posteo.de>
Signed-off-by: Guido Günther <agx@sigxcpu.org>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Stefan Agner <stefan@agner.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/781f0352052cc50c823c199ef5f53c84902d0580.1633959458.git.agx@sigxcpu.org