Change to use correct check for halfpel

Change to use correct check for halfpel

In motion estimation stage for subpel motion, subpel variance is
computed use bilinear interpolation. The motion vector precision
used is at 1/8 pel and three bits are used to represent the x and y
subpel offsets. Based on this, the half pel check should be against
4, not 8.

Change-Id: I1f56fa1fa3f2f5e19a20d27983efe628557f170e