Optimize the scaling calculation

In decoder, the scaling calculation, such as (mv * x_num / x_den),
is fairly time-consuming. In this patch, we check if the scaling
happens or not at frame level, and then decide which function to
call to skip scaling calculation when no scaling is needed. Tests
showed a 3% decoder performance gain.

Change-Id: I270901dd0331048e50368cfd51ce273dd82b8733