drm: mxsfb: fix fence synchronization

The conversion away from the simple display pipeline helper missed
to convert the prepare_fb plane callback, so no fences are attached to
the atomic state, breaking synchronization with other devices. Fix
this by plugging in the drm_gem_fb_prepare_fb helper function.

Fixes: ae1ed009 ("drm: mxsfb: Stop using DRM simple display pipeline helper")
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Stefan Agner <stefan@agner.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201120211306.325841-1-l.stach@pengutronix.de