Restructure vp9_decode_mb_tokens_16x16() a bit.

Don't declare variables if they only ever have a single value and are
used only as argument to another function call; instead, just hardcode
the value in the function call directly. Also remove unneeded brackets
around a code block, and remove the magic offsets 64 and 256 for chroma
values in the coefficient memory block.

Change-Id: I14fc14120a81ea1d6fb862674e8bf8cf6ba3d114