This change is similar to the one done for choose_tx_size_from_rd in
daf841b4

It gives a 4% speed-up on bus_cif.y4m with the following settings:
--cpu-used=4 -p 1 --end-usage=q --cq-level=40 --tile-columns=0 --tile-rows=0

Change-Id: Ic54fe4a066a2c0b5f6349d80cd13de8bb8ddcabc