Currently, optimize_txb does not use the new-quant
lookup table when it updates the coefficients. This
fixes the function to avoid a mismatch.

With this patch, new-quant adds 0.28% gains on top of
lv-map-multi on 40 frames of lowres.

Change-Id: Ic4e76c28c7e4e007d277fe92b9030e078768bab3