Refactor vp10_xform_quant

1) Add facade to quantize b/fp/dc version so that their interface
   are the same.
2) Merge vp10_xform_quant b/fp/dc version to one function so that
   the code flow in encodemb.c is clear

Change-Id: Ib62d6215438fc2d07f4e7e72393f964832d6746f
7 files changed