drm/i915: Hold mode_config lock whilst changing mode for lastclose()

Upon lastclose(), we switch back to the fbcon configuration. This
requires taking the mode_config lock in order to serialise the change
with output probing elsewhere.

Reported-by: Oleksij Rempel <bug-track@fisher-privat.net>
References: https://bugs.freedesktop.org/show_bug.cgi?id=48652
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@kernel.org
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_fb.c

@@ -279,6 +279,8 @@ void intel_fb_restore_mode(struct drm_device *dev)
 	struct drm_mode_config *config = &dev->mode_config;
 	struct drm_plane *plane;
 
+	mutex_lock(&dev->mode_config.mutex);
+
 	ret = drm_fb_helper_restore_fbdev_mode(&dev_priv->fbdev->helper);
 	if (ret)
 		DRM_DEBUG("failed to restore crtc mode\n");
@@ -286,4 +288,6 @@ void intel_fb_restore_mode(struct drm_device *dev)
 	/* Be sure to shut off any planes that may be active */
 	list_for_each_entry(plane, &config->plane_list, head)
 		plane->funcs->disable_plane(plane);
+
+	mutex_unlock(&dev->mode_config.mutex);
 }