Each CDF for partitioning square blocks is initialised from
an entry of default_partition_cdf in entropymode.c. These CDFs are of
different lengths, depending on which partition types are supported by
the block size.

For example, 8x8 blocks have a CDF with only 4 entries (PARTITION_NONE
through PARTITION_SPLIT). Blocks of a size that supports 1:4 and 4:1
partitions have 10 entries. Currently, that's only 32x32 blocks. All
other blocks have 8 entries.

Change-Id: Ie2126b6d41afc0efedcc5b5b37fc1d0427b9a9fa