Use non-zero mode threshold for NEARESTMV modes

This commit makes the encoder to use non-zero mode threshold for
NEARESTMV modes. The runtime for test clips of speed 3 is reduced
by about 1%.

pedestrian 1080p 2000 kbps, 143239 ms -> 141989 ms
bus CIF 1000 kbps, 7835 ms -> 7749 ms

The compression performance change is about -0.02% for both derf
and stdhd.

Change-Id: Ib71808922c41ae2997100cb7c561f68dcebfa08e