1) move the original implementation in av1_cost_coeffs() to
cost_coeffs() and let av1_cost_coeffs become a switch for
choosing original coeff cost or lv_map's coeff cost

2) change get_txb_ctx's naming. Use plane_bsize instead of
bsize to make the intention clear.

3) remove txb context computing in get_txb_ctx
Change-Id: I17e3d39d796e051d1c90f0a0c5d7d0888b9ca292