fix a problem where an invalid mv used in search

The commit added reset of pred_mv at the beginning of each SB64x64
partition mv search, also limited the usage of pred_mv only when
search on the largest partition is already done. This is to fix
a crash at speed 1/2 encoder where an invalid mv is used in mv
search.

Change-Id: I39010177da76d054e3c90b7899a44feb2e3a5b1b