freedreno: Drop the extra offset field for mipmap slices.

freedreno: Drop the extra offset field for mipmap slices.

We can just bake the UBWC-goes-first delta into the slices at setup time.
I did have to fix up the resource shadowing swap path to swap the slice
fields, as it was missing and regressed the format reinterpets otherwise.

Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>