drm: Check locking in drm_for_each_connector

Because of DP MST connectors can now be hotplugged and we must hold
the right lock when walking the connector lists.  Enforce this by
checking the locking in our shiny new list walking macros.

v2: Extract the locking check into a small static inline helper to
help readability. This will be more important when we make the
read list access rules more complicated in later patches. Inspired by
comments from Chris. Unfortunately, due to header loops around the
definition of struct drm_device the function interface is a bit funny.

v3: Encoders aren't hotadded/removed. For each dp mst encoder we
statically create one fake encoder per pipe so that we can support as
many mst sinks as the hw can (Dave).

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

include/drm/drm_crtc.h

@@ -1589,8 +1589,18 @@ static inline struct drm_property *drm_property_find(struct drm_device *dev,
 #define drm_for_each_crtc(crtc, dev) \
 	list_for_each_entry(crtc, &(dev)->mode_config.crtc_list, head)
 
+static inline void
+assert_drm_connector_list_read_locked(struct drm_mode_config *mode_config)
+{
+	WARN_ON(!mutex_is_locked(&mode_config->mutex));
+}
+
 #define drm_for_each_connector(connector, dev) \
-	list_for_each_entry(connector, &(dev)->mode_config.connector_list, head)
+	for (assert_drm_connector_list_read_locked(&(dev)->mode_config),	\
+	     connector = list_first_entry(&(dev)->mode_config.connector_list,	\
+					  struct drm_connector, head);		\
+	     &connector->head != (&(dev)->mode_config.connector_list);		\
+	     connector = list_next_entry(connector, head))
 
 #define drm_for_each_encoder(encoder, dev) \
 	list_for_each_entry(encoder, &(dev)->mode_config.encoder_list, head)