drm/fb-helper: Fix locking in drm_fb_helper_hotplug_event

Driver's and ->fill_modes functions are allowed to grab crtc mutexes
(for e.g. load detect). Hence we need to first only grab the general
kms mutex, and only in a second step grab all locks to do the
modesets.

This prevents a deadlock on my gm45 in the tv load detect code called
by drm_helper_probe_single_connector_modes.
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_fb_helper.c

@@ -1544,10 +1544,10 @@ int drm_fb_helper_hotplug_event(struct drm_fb_helper *fb_helper)
 	if (!fb_helper->fb)
 		return 0;
 
-	drm_modeset_lock_all(dev);
+	mutex_lock(&fb_helper->dev->mode_config.mutex);
 	if (!drm_fb_helper_is_bound(fb_helper)) {
 		fb_helper->delayed_hotplug = true;
-		drm_modeset_unlock_all(dev);
+		mutex_unlock(&fb_helper->dev->mode_config.mutex);
 		return 0;
 	}
 	DRM_DEBUG_KMS("\n");
@@ -1558,9 +1558,11 @@ int drm_fb_helper_hotplug_event(struct drm_fb_helper *fb_helper)
 
 	count = drm_fb_helper_probe_connector_modes(fb_helper, max_width,
 						    max_height);
+	mutex_unlock(&fb_helper->dev->mode_config.mutex);
+
+	drm_modeset_lock_all(dev);
 	drm_setup_crtcs(fb_helper);
 	drm_modeset_unlock_all(dev);
-
 	drm_fb_helper_set_par(fb_helper->fbdev);
 
 	return 0;