With --enable-ext-partition, it is possible to have superblock size at
both 64x64 and 128x128. But MAX_SB_SIZE_LOG2 is defined to be 7, i.e.
128x128 size. In get_token_alloc(), the token count for a tile or a
frame is always computed based on "tokens/superblock * number of
superblocks", so it should use the actual superblock size, instead of
128x128.

The segfault in #988 is a result when every tile is defined at one
single superblock of 64x64, the token size computation for each tile
was assuming it is 128x128, resulting 4x overestimate of the token
count for every tile. This caused the pointer for many tiles to go
beyond the allocated token arrary, which is correctly allocated at
frame level.

BUG=aomedia:988

Change-Id: I2da21f181446ad7117ce3269f01a43950c379028