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