util: Make all 3 fetch_rgba functions occupy the same function slot.

util: Make all 3 fetch_rgba functions occupy the same function slot.

A single format either had the float, the sint, or the uint version.
Making the dst be void * lets us store them in the same slot and not have
logic in the callers to call the right one.

-6kb on gallium drivers

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6305>