Remove costly memory reads/writes in vp8_reset_mb_tokens_context()
Remove costly memory reads/writes in vp8_reset_mb_tokens_context()

Tests on x86 showed this function costed 2.7% of total decoding time
because of all the memory reads/writes. After modification, it only
costs about 0.7% of decoding time, which gives a 2% gain.

Change-Id: I5003ee30b6dc6dea0bfa42a6ad7e7c22fcc7b215

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