"qc" in vp{9,10}_token_state is used to save quantized coefficients, this
commit changes the type from short to tran_low_t to properly reflect
the value range for highbitdepth build.

This fixes an out-of-range bug when optimize_b is used in highbitdepth
build.

Change-Id: Ibf330879e6ac6ae8f099e085caa9d3d9a889fde8