drm/crtc: Use drm_mode_object_put() in __drm_framebuffer_unregister()

The function __drm_framebuffer_unregister() has boilerplate code to drop idr
reference.  Let's replace it with drm_mode_object_put() to simplify the code.
Signed-off-by: Liu Ying <gnuiyl@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1456716070-2602-1-git-send-email-gnuiyl@gmail.com

drivers/gpu/drm/drm_crtc.c

@@ -430,9 +430,7 @@ EXPORT_SYMBOL(drm_framebuffer_init);
 static void __drm_framebuffer_unregister(struct drm_device *dev,
 					 struct drm_framebuffer *fb)
 {
-	mutex_lock(&dev->mode_config.idr_mutex);
-	idr_remove(&dev->mode_config.crtc_idr, fb->base.id);
-	mutex_unlock(&dev->mode_config.idr_mutex);
+	drm_mode_object_put(dev, &fb->base);
 
 	fb->base.id = 0;
 }