drm/i915: Assert that the handle->vma lut is empty on object close

Make sure that we are not leaking an entry in the ctx->handles_lut by
asserting that the object was removed prior to being freed. This should
be enforced by all such handles being removed by i915_gem_close_object.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20170822110517.22277-2-chris@chris-wilson.co.ukReviewed-by: Michał Winiarski <michal.winiarski@intel.com>

drivers/gpu/drm/i915/i915_gem.c

@@ -4416,6 +4416,7 @@ static void __i915_gem_free_objects(struct drm_i915_private *i915,
 	llist_for_each_entry_safe(obj, on, freed, freed) {
 		GEM_BUG_ON(obj->bind_count);
 		GEM_BUG_ON(atomic_read(&obj->frontbuffer_bits));
+		GEM_BUG_ON(!list_empty(&obj->lut_list));
 
 		if (obj->ops->release)
 			obj->ops->release(obj);