vp9_f(dct|ht): disable avx2 variants

tests failing under Win32/Win64

+ dct16x16_test: add missing avx2 functions (partially disabled)

exercises the forward transforms
no idct/iht implementations, so the c-code is used

Change-Id: I04f64a457fa0828a00f32b5c9fe4f55294f21f61