Restructure vp9_decode_mb_tokens_8x8() 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. Split out UV and Y coefficient
loops for clarity. Use xd->block[].qcoeff instead of xd->qcoeff + magic
to remove use of magic offset variables.

Change-Id: I5b17eda1bb666c69c2b7ea957d5525cd78192e33