MLK-16918-7: drm/mxsfb: Update mxsfb to support a bridge

MLK-16918-7: drm/mxsfb: Update mxsfb to support a bridge

Currently, the MXSFB DRM driver only supports a panel. But, its output
display signal can also be redirected to another encoder, like a DSI
controller. In this case, that DSI controller may act like a drm_bridge.
In order support this use-case too, this patch adds support for
drm_bridge in mxsfb.

Signed-off-by: Robert Chiras <robert.chiras@nxp.com>