This commit moves the simplified coefficient probability model
and costing update to speed 4, and turns on chessboard pattern
mode search for sub 720p sequences. The overall coding performance
of speed 3 is improved:
derf  0.889%
stdhd 1.744%

The speed 3 runtime for test sequences are improved:
bus cif at 1000 kbps 9823 ms -> 9642 ms
pedestrian 1080p 2000 kbps 189559 ms -> 183284 ms

Change-Id: Iecbc7496a68f31fd49fb09f8dfd97c028d675a5d