Add functions drm_fb_cma_fbdev_init(), drm_fb_cma_fbdev_fini() and
drm_fb_cma_fbdev_init_with_funcs(). These functions relies on the fact
that the drm_fb_helper struct is stored in dev->drm_fb_helper_private
so drivers don't need to store it.

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20171115142001.45358-3-noralf@tronnes.org