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: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@linux.ie>
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index ff0d94d..f565b6a 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -758,8 +758,11 @@
 
 	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;