Prune single ref modes based on mv difference and mode rate

Prune single ref modes based on mv difference and mode rate

This patch introduces a speed feature to prune single reference
modes - NEARMV and ZEROMV based on motion vector difference and
mode rate w.r.t previously evaluated single reference modes
corresponding to the same reference frame.

                Instruction Count        BD-Rate Loss(%)
cpu   Resolution   Reduction(%)    avg.psnr   ovr.psnr    ssim
 0       LOWRES2      1.686        -0.0039    -0.0105   -0.0098
 0       MIDRES2      1.026        -0.0234     0.0029    0.0120
 0        HDRES2      0.000         0.0000     0.0000    0.0000
 0       Average      0.889        -0.0091    -0.0025    0.0007

STATS_CHANGED

Change-Id: I387acd3a73d8256904a7ce684b198d251cf3dd04