drm/i915: Release and unlock on mmap_gtt error path.

We failed to unlock the mutex after failing to create the mmap offset.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NEric Anholt <eric@anholt.net>
Signed-off-by: NDave Airlie <airlied@linux.ie>

drivers/gpu/drm/i915/i915_gem.c

@@ -758,8 +758,11 @@ i915_gem_mmap_gtt_ioctl(struct drm_device *dev, void *data,
 
 	if (!obj_priv->mmap_offset) {
 		ret = i915_gem_create_mmap_offset(obj);
-		if (ret)
+		if (ret) {
+			drm_gem_object_unreference(obj);
+			mutex_unlock(&dev->struct_mutex);
 			return ret;
+		}
 	}
 
 	args->offset = obj_priv->mmap_offset;