vp9/x86: fix bug in intra_pred_hd_32x32.

Fixes mismatch in first keyframe in sample
ffvp9_fails_where_libvpx.succeeds.webm from ticket 3849. There's still
a second mismatch a few frames into the sample.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>