Modified the  mv_pred() fuunction that chooses a centre
point from which to start step searches to use the top
candidate vectors chosen previously.

Some gains (mainly on HD and tested with SB off).
Std_hd 0.874%, YT-hd 0.174%, YT 0.05%, Derf 0.036%

Change-Id: Ie232284f561838b8ecee0e28dcbb07a9cd46cf56