This adds a .fb_create helper that sets the .dirty callback to
drm_atomic_helper_dirtyfb().

v2: Improve docs (Daniel)
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190115043643.2364-2-noralf@tronnes.org