This patch enables the skip prediction feature for blocks larger than
16x16; but the tx size used in this module is still limited to 16x16.

Also makes it exit early upon large quantized coeff.

Test results:
-encoding speed up by 0~17%, on average 8.6%, based on several lowres
test sequences;
-no quality loss observed, encoding 30 frames on lowres and midres.

BUG=aomedia:1106

Change-Id: Ic2051a978eda5c8ce6a795a9d97d8d114c6edfe2