drm/mxsfb: Read bus flags from bridge if present

The bridge might have special requirmentes on the input bus. This
is e.g. used by the imx-nwl bridge.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Reviewed-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Stefan Agner <stefan@agner.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1567078215-31601-3-git-send-email-robert.chiras@nxp.com