Rewrite coeffs_kpp_low to mimic POP calls

To make it easier to compare POP and POP+CVMix, and to help debug, I rewrote
the cvmix_coeffs_kpp_low routine to do things in the same order as POP. As far
as I can tell, all the level indexes are correct (tracking the differences
introduced because POP indexes interfaces beginning at the bottom was a pain,
but it looks like it's all okay).