pixel: fix avx2, 16bpp, and testbench following luma enum reorg
intra: remove unused argument to predDCFiltering(), remove static

There's no need to declare the function static, it is within an anonymous
namespace
intra: segregate 8bpp from 16bpp functions, drop 16bpp angular, drop 64x64

The HIGH_BIT_DEPTH angular function was just a copy of the C reference, we do
not need 64x64 blocks any more