drm/i915: Call i915_gem_gtt_fini() when the device is detached

This fixes several memory leaks. Apparently, this problem exists in
Linux 3.8 but the code changed in Linux 3.9 so it may be fixed upstream
already. Still, this is something we need to pay attention to.

sys/dev/drm2/i915/i915_dma.c

@@ -1810,6 +1810,12 @@ int i915_driver_unload(struct drm_device *dev)
 	if (dev_priv->mmio_map != NULL)
 		drm_rmmap(dev, dev_priv->mmio_map);
 
+	/*
+	 * NOTE Linux<->FreeBSD: Linux forgots to call
+	 * i915_gem_gtt_fini(), causing memory leaks.
+	 */
+	i915_gem_gtt_fini(dev);
+
 	if (dev_priv->wq != NULL)
 		taskqueue_free(dev_priv->wq);