Simplify & fix potential bug in rd_pick_partition

Different partitionings were not being evaluated against
best_rd and there were unnecessary calls to RDCOST. This
could have resulted in a non-optimal partioning being
selected.

I simplified the variables used to track the rate,
distortion and RD values throughout the function.

Change-Id: Ifa7085ee80d824e86791432a5bc6d8fea5a3e313