drm/i915: Takedown drm_mm on failed gtt setup

This was found by code inspection. If the GTT setup fails then we are
left without properly tearing down the drm_mm.

Hopefully this never happens.
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -4508,6 +4508,7 @@ int i915_gem_init(struct drm_device *dev)
 	mutex_unlock(&dev->struct_mutex);
 	if (ret) {
 		i915_gem_cleanup_aliasing_ppgtt(dev);
+		drm_mm_takedown(&dev_priv->gtt.base.mm);
 		return ret;
 	}