freedreno/a6xx: Fix the size of buffer image views.

freedreno/a6xx: Fix the size of buffer image views.

We were using the size of the underlying buffer (in R8 elements), while we
need to be using the size of the image view (which may be a subset of the
underlying buffer, and may be in a different format from R8).

This fix means less dereferencing off of the end of shader image views for
buffer images, but more importantly is needed to get the right answer from
resinfo if we are to switch to that.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3501>