drm: Amend connector list locking rules

Now that dp mst hotplug takes all locks we can amend the locking rules
for the iterators. This is needed before we can roll these out in the
atomic code to avoid getting burried in WARNINGs.

v2: Rebase onto the extracted list locking assert and add a comment to
explain the rules.

v3: Fixup German->English translation fail in the comment.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NMaarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@intel.com>

include/drm/drm_crtc.h

@@ -1592,7 +1592,15 @@ static inline struct drm_property *drm_property_find(struct drm_device *dev,
 static inline void
 assert_drm_connector_list_read_locked(struct drm_mode_config *mode_config)
 {
-	WARN_ON(!mutex_is_locked(&mode_config->mutex));
+	/*
+	 * The connector hotadd/remove code currently grabs both locks when
+	 * updating lists. Hence readers need only hold either of them to be
+	 * safe and the check amounts to
+	 *
+	 * WARN_ON(not_holding(A) && not_holding(B)).
+	 */
+	WARN_ON(!mutex_is_locked(&mode_config->mutex) &&
+		!drm_modeset_is_locked(&mode_config->connection_mutex));
 }
 
 #define drm_for_each_connector(connector, dev) \