Enable dynamic motion vector referencing for newmv mode

This commit enables the dynamic motion vector predictor for NEWMV
mode. It allows the codec to select the best motion vector predictor
in a rate-distortion optimization framework for motion vector
residual coding. The compression performance is improved:
lowres  0.14%
midres  0.27%
hdres   0.24%

Change-Id: I6a601c74eb6cb0b71a613336d40363359f2edecd