Reduce the frequency of partition size adjsutment

The commit change to use partitions sizes directly from last frame
for frames directly where last frame selects partitions sizes based
on coding efficiency.

On --rt --cpu-used=-5, the change hurts compression by 4% but reduces
encoding time by ~20%

Change-Id: Ia68665e5c8489b7bfcf5fac7768332fba88928e6