Disable vpx_highbd_8_mse16x16_neon_dotprod, etc.

Disable vpx_highbd_8_mse16x16_neon_dotprod, etc.

These functions assume the uint16_t samples are <= 255 (bit depth 8),
but vpx_highbd_8_mse16x16() is called for any bit depth, not just 8.

A better fix is to port the libaom CL
https://aomedia-review.googlesource.com/c/aom/+/175063 to libvpx, but
that requires porting aom_sse() and aom_highbd_sse() to libvpx, which is
quite involved. So disable vpx_highbd_8_mse16x16_neon_dotprod, etc.
first.

Bug: webm:1819
Change-Id: If495a5dedc58d9981317b9993c9fbb81ff3ab50c