drm: mxsfb: Remove fbdev leftovers

Commit 8e93f102 ("drm/mxsfb: Use drm_fbdev_generic_setup()")
replaced fbdev handling with drm_fbdev_generic_setup() but left
inclusion of the drm/drm_fb_cma_helper.h header. Remove it.

Fixes: 8e93f102 ("drm/mxsfb: Use drm_fbdev_generic_setup()")
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/20200727020654.8231-2-laurent.pinchart@ideasonboard.com