drm/connector: Make ->atomic_commit() optional

Not all writeback connector implementations might want to commit things
from the connector driver. Some, like the malidp driver, commit things
from their main commit_tail() function, and would rather not have to
implement a dummy hook for drm_connector_helper_funcs.atomic_commit().

Make this function optional and reflect this fact in the doc.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180703075022.15138-4-boris.brezillon@bootlin.com

include/drm/drm_modeset_helper_vtables.h

@@ -983,6 +983,8 @@ struct drm_connector_helper_funcs {
 	 * The writeback_job to commit is available in
 	 * &drm_connector_state.writeback_job.
 	 *
+	 * This hook is optional.
+	 *
 	 * This callback is used by the atomic modeset helpers.
 	 */
 	void (*atomic_commit)(struct drm_connector *connector,