drm/fb-cma-helper: Add function drm_fb_cma_create_with_funcs()

Add drm_fb_cma_create_with_funcs() for drivers that need to set the
dirty() callback.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1463077523-23959-3-git-send-email-noralf@tronnes.org