vp10: only assume ONLY_4X4 if segmentation is disabled.

Otherwise, per-segment lossless might mean that some segments are not
lossless and they could still want to use another mode. The per-block
tx points remain uncoded on blocks where (per the segment id) the Q
value implies lossless.

Change-Id: If210206ab1fe3dd11976797370c77f961f13dfa0