Restrict ref mv search range.

Experiment to test speed trade off of reducing the
extent of the ref mv search.

Reducing the maximum number of tested candidates to 9 had
minimal net effect on quality in any of the tests sets.

Reduction to 7 has a small negative impact (worst was STD-HD
at about -0.2%).

This change is in response to the apparently high number of
decode cycles reported in regard to mv-ref selection.

Change-Id: I0e92e92e324337689358495a1ec9ccdeb23dc774