zink: fixup sampler-usage

zink: fixup sampler-usage

It seems I got this stuff all wrong, and looked at driver_location
rather than the binding. But since we mess with the binding, we need to
adjust things a bit to get things right.

This still isn't great as-is, but it seems to work. In the future, we
should move to having samplers always at bindings 0 and up, and just
update the bindings that are used by either of the stages. But this
band-aid should be OK for now.

This fixes 0AD for me.

Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3668>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3668>