Make memory alloc in pick_mode_context bsize aware

This commit makes the buffer allocation of zcoeff_blk array in
pick_mode_context block size aware. It calculates the number of
4x4 blocks in the partition and assigns the memory space accordingly.
This process (and the uninitialization) is done once for each encoding
pass. It allows memory copy of smaller buffer when possible.

For football at 600kbps, the runtimes improve by about 1%:
speed 1, 45961ms -> 45472ms
speed 2, 23863ms -> 23598ms

Change-Id: Id2ca24906fa89f46fa5fe742ec4b8efc2a61f877