This probably has a mildly negative impact on performance, but will
(in future commits - or possibly merged with this one) allow SIMD
implementations of individual intra prediction functions. We may
perhaps want to consider having separate functions per txfm-size
also (i.e. 4x4, 8x8, 16x16 and 32x32 intra prediction functions for
each intra prediction mode), but I haven't played much with that
yet.

Change-Id: Ie739985eee0a3fcbb7aed29ee6910fdb653ea269