Move token_cache from cost_coeffs to MACROBLOCK

This commit moves token_cache buffer into macroblock struct, instead
of defining as a local variable in cost_coeffs. This avoids repeatedly
re-allocating memory space in the rate-distortion optimization loop.

The runtime at speed 0 reduces:
bus 2000kbps, 161692ms to 159951ms
football 600kbps, 229505ms to 225821ms

Change-Id: If7da6b0b6d8c5138a16271a33c4548fba33d8840