one level less of tx size search for blocks larger than 64
one level less of tx size search for blocks larger than 64

3~5% encoding speedup for speed 0; no quality loss.

Change-Id: I0e31755f45253e5e99d8d9eed0d7a6fe6050f49f

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom