When calling aom_(highbd_)blend_a64_mask for a subsampled plane,
the mask is subsampled in-place. This means that the mask stride
passed in needs to be the value before subsampling. Since the
wedge masks are set up so that mask stride = mask width, this means
that the stride should equal the width of the current mi block.

However, for the wedge-interintra path in combine_interintra_highbd,
we subsampled the mask stride, resulting in the wrong stride being
used for chroma planes if subsampling_x == 1.

The lowbd path already uses the correct stride, so this patch just
copies the stride calculation over to the highbd path.

Change-Id: I08a4120b11a38912c45d9e637a54e009eddd88ec