Turn on ml_var_partition_pruning for HBD
Turn on ml_var_partition_pruning for HBD

This affects speed 0 and 1 only.

Tested on lowres_bd10(480p) and midres_bd10(720p),
                   speed 0       speed 1
coding loss:        0.07%         0.10%
encoder speedup:     14%          6.5%

Change-Id: I5812400d8c7393321b7284d3fca06026842390b5

webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx