This commit allows the dynamic motion vector referencing system to
scale its search range according to the coding block size. This
provides higher search resolution for smaller size coding unit.

The cb4x4 mode improves the compression performance across all the
test sets:

         avg     low    mid    high
lowres   2.8%    2.4%   3.1%   3.0%
midres   1.3%    0.3%   1.8%   2.7%
hdres    0.9%    0.5%   1.4%   1.5%

Change-Id: I1bc501506a9f2f06071c5274391f6bd053b235a7