drm/ttm: don't oops if no invalidate_caches()
drm/ttm: don't oops if no invalidate_caches()

From Rob Clark
8af4707afcb6d191b9cc75dfd73b9550e8276253 in ubuntu 3.8
9ef7506f7eff3fc42724269f62e30164c141661f in mainline linux

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars