av1: use chroma txtp inference over default DCT_DCT if qidx=0

Fixes #320. The problem here is that qidx=0 is (in libaom) a shortcut
for lossless, which normally becomes WHT_WHT, but under some obscure
conditions, it can also be non-lossless, in which case qidx=0 implies
DCT_DCT for luma. For chroma, apparently we should use the default
inference pattern, which becomes DCT_DCT for inter also, but requires
the standard lookup table for intra.