Skip buffer update in sub8x8 rd loop

This commit allows the encoder to skip a few buffer update steps in
rd_pick_best_mbsegmentation, when early breakout has been triggered
in the rd_check_segment_txsize. It provides about 1% speed-up for
bus_cif at 2000 kbps, in the settings of speed 0.

Change-Id: Ica034f10a24dec572b397d8389a2b81020ebc0b9