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
1 file changed