drm/i915: Reset vma->mm_list after unbinding

In place of true activity counting, we walk the list of vma associated
with an object managing each on the vm's active/inactive list everytime
we call move-to-inactive. This depends upon the vma->mm_list being
cleared after unbinding, or else we run into difficulty when tracking
the object in multiple vm's - we see a use-after free and corruption of
the mm_list.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Cc: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: NBen Widawsky <ben@bwidawsk.net>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -2739,7 +2739,7 @@ int i915_vma_unbind(struct i915_vma *vma)
 
 	i915_gem_gtt_finish_object(obj);
 
-	list_del(&vma->mm_list);
+	list_del_init(&vma->mm_list);
 	/* Avoid an unnecessary call to unbind on rebind. */
 	if (i915_is_ggtt(vma->vm))
 		obj->map_and_fenceable = true;