When the prediction residuals are all zero, reset the coeff rate
cost and the distortion value to be zero. This change doesn't affect
lowres set significantly, but improves several clips in the midres
set, like sintel_480p and mobisode2_480p, by a few percents. The
average performance for midres set is improved by 0.2%.

Change-Id: Idd5ebf2652e556a1b1c569fe3c48dacef3f11c32