Stop partition checking when distortion is small

If the current obtained distortion is very small, which happens
for static image case, we pick the current partition type without
further split checking.

This won't affect regular videos. For static videos, we got 10%~12%
encoding speed gain. PSNR was better for some clips, and worse for
others. Overall it was even.

Change-Id: If787a57bedf46fc595ca4f5ded2b0c0a69e9fdef