drm: mxsfb: Stop using DRM simple display pipeline helper

The DRM simple display pipeline helper only supports a single plane. In
order to prepare for support of the alpha plane on i.MX6SX and i.MX7,
move away from the helper. No new feature is added.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Stefan Agner <stefan@agner.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200727020654.8231-12-laurent.pinchart@ideasonboard.com