ttm use pages don't need to populate?

[Yuan, Shengquan]

ttm_tt_set_user has "ttm->state = tt_unbound" which will cause the
following ttm_tt_populate return directly because of bellow check:
        if (ttm->state != tt_unpopulated)
                return 0;

Does it mean we don't need to populate user pages?  Here is the patch
if it is not the case

Thanks
Austin