drm: mxsfb: Remove unneeded includes

A fair number of includes are not needed. Drop them, and add a couple of
required includes that were included indirectly.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Stefan Agner <stefan@agner.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200727020654.8231-10-laurent.pinchart@ideasonboard.com