libavcodec/ppc/mpegvideoencdsp.c: fix stack smashing in pix_norm1_altivec() and pix_sum_altivec()

The vec_ste calls were mistakenly changed to vec_vsx_st in c5ca76a, which
caused stack smashing.

Changing them back fixes crashes on ppc64el, when configured with
--toolchain=hardened.

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