drm: don't cast a pointer to pointer of list_head

The casting is safe only when the list_head member is the first member of
the structure.
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dave Airlie <airlied@linux.ie>

drivers/char/drm/i915_irq.c

@@ -555,7 +555,7 @@ int i915_vblank_swap(struct drm_device *dev, void *data,
 
 	spin_lock_irqsave(&dev_priv->swaps_lock, irqflags);
 
-	list_add_tail((struct list_head *)vbl_swap, &dev_priv->vbl_swaps.head);
+	list_add_tail(&vbl_swap->head, &dev_priv->vbl_swaps.head);
 	dev_priv->swaps_pending++;
 
 	spin_unlock_irqrestore(&dev_priv->swaps_lock, irqflags);