drm: mxsfb: Support the alpha plane

The LCDIF in the i.MX6SX and i.MX7 have a second plane called the alpha
plane. Support it.

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/20200727020654.8231-23-laurent.pinchart@ideasonboard.com