gallium/util: Clean up the Z/S tile write path.

The switch statement is silly, we have a helper to detect all of Z/S.
And, take the time explain why all of Z/S tile writing is unimplemented.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5728>
1 file changed