drm/i915: Fix object refcount leak on mmappable size limit error path.

I've been seeing memory leaks on my system in the form of large
(300-400MB) GEM objects created by now-dead processes laying around
clogging up memory.  I usually notice when it gets to about 1.2GB of
them.  Hopefully this clears up the issue, but I just found this bug
by inspection.
Signed-off-by: Eric Anholt <eric@anholt.net>
Cc: stable@kernel.org
Signed-off-by: Keith Packard <keithp@keithp.com>

drivers/gpu/drm/i915/i915_gem.c

@@ -1396,7 +1396,7 @@ i915_gem_mmap_gtt(struct drm_file *file,
 
 	if (obj->base.size > dev_priv->mm.gtt_mappable_end) {
 		ret = -E2BIG;
-		goto unlock;
+		goto out;
 	}
 
 	if (obj->madv != I915_MADV_WILLNEED) {