The rd_thresholds are adaptively changed based on best mode tested.
It was only changed for the same block size, this commit makes the
adaptation for similar block sizes too. The commit also made minor
adjustment and code cleanups.

The impact on encoding time for _ped:
118089 ms -> 111927 ms

The impact on compression:
derf:  -0.339%
stdhd: -0.303%

Change-Id: I8817fed1102350497f2ec631849e43f753878e5d