Removing rounding from UV MV calculation

Consider the previous behavior for the MV 1 3/8 (11/8 pel). In the
existing code, the fractional part of the MV is considered separately,
and rounded is applied, giving a result of 6/8. Rounding is not required
in this case, as we're increasing the precision from a q3 to a q4, and
the correct value 11/16 can be represented exactly.

Slight gain observed (+.033 average on derf)

Change-Id: I320e160e8b12f1dd66aa0ce7966b5088870fe9f8