texture_cache/util: Fix size calculations of multisampled images

On the texture cache we handle multisampled images by keeping their real
size in samples (e.g. 1920x1080 with 4 samples is 3840x2160).

This works nicely with size matches and other comparisons, but the
calculation for guest sizes was not having this in mind, and the size
was being multiplied (again) by the number of samples per dimension.
For example a 3840x2160 texture cache image had its width and height
multiplied by 2, resulting in a much larger texture.

Fix this issue.

- Fixes performance regression on cooking related titles when an
  unrelated bug was fixed.