drm/atomic: Fix remaining places where !funcs->best_encoder is valid

Adapt drm_pick_crtcs() and update_connector_routing() to fallback to
drm_atomic_helper_best_encoder() if funcs->best_encoder() is NULL so
that DRM drivers can leave this hook unassigned if they know they want
to use drm_atomic_helper_best_encoder().

Update the vtables documentation accordingly.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1465300095-16971-2-git-send-email-boris.brezillon@free-electrons.com
diff --git a/include/drm/drm_modeset_helper_vtables.h b/include/drm/drm_modeset_helper_vtables.h
index d4619dc..4e7a53b 100644
--- a/include/drm/drm_modeset_helper_vtables.h
+++ b/include/drm/drm_modeset_helper_vtables.h
@@ -736,6 +736,11 @@
 	 * inspect dynamic configuration state should instead use
 	 * @atomic_best_encoder.
 	 *
+	 * You can leave this function to NULL if the connector is only
+	 * attached to a single encoder and you are using the atomic helpers.
+	 * In this case, the core will call drm_atomic_helper_best_encoder()
+	 * for you.
+	 *
 	 * RETURNS:
 	 *
 	 * Encoder that should be used for the given connector and connector
@@ -752,8 +757,9 @@
 	 * need to select the best encoder depending upon the desired
 	 * configuration and can't select it statically.
 	 *
-	 * This function is used by drm_atomic_helper_check_modeset() and either
-	 * this or @best_encoder is required.
+	 * This function is used by drm_atomic_helper_check_modeset().
+	 * If it is not implemented, the core will fallback to @best_encoder
+	 * (or drm_atomic_helper_best_encoder() if @best_encoder is NULL).
 	 *
 	 * NOTE:
 	 *