Correct the mode decisions in special cases

The rate costs calculated for inter modes are not precise in some
cases, which causes NEWMV is chosen instead of NEARESTMV, NEARMV,
and ZEROMV. This patch added checks for these cases, and corrected
the mode decisions.

Borg tests at speed 3 showed:
1. stdhd set: 0.102% PSNR gain and 0.088% SSIM gain.
2. derf set:  0.147% PSNR gain and 0.132% SSIM gain.
No speed change.

Change-Id: I35d17684b89ad4734fb610942d707899146426db