drm/nouveau: add lockdep annotations

1) Lockdep thinks all nouveau subdevs belong to the same class and can be
locked in arbitrary order, which is not true (at least in general case).
Tell it to distinguish subdevs by (o)class type.
2) DRM client can be locked under user client lock - tell lockdep to put
DRM client lock in a separate class.

Reported-by: Arend van Spriel <arend@broadcom.com>
Reported-by: Peter Hurley <peter@hurleysoftware.com>
Reported-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Reported-by: Daniel J Blueman <daniel@quora.org>
Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Cc: stable@vger.kernel.org [3.7, but needs s/const ofuncs/ofuncs/ to build]
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_drm.c

@@ -245,6 +245,8 @@ nouveau_drm_probe(struct pci_dev *pdev, const struct pci_device_id *pent)
 	return 0;
 }
 
+static struct lock_class_key drm_client_lock_class_key;
+
 static int
 nouveau_drm_load(struct drm_device *dev, unsigned long flags)
 {
@@ -256,6 +258,7 @@ nouveau_drm_load(struct drm_device *dev, unsigned long flags)
 	ret = nouveau_cli_create(pdev, "DRM", sizeof(*drm), (void**)&drm);
 	if (ret)
 		return ret;
+	lockdep_set_class(&drm->client.mutex, &drm_client_lock_class_key);
 
 	dev->dev_private = drm;
 	drm->dev = dev;