zink: fix enabled vertex buffer mask calculation

zink: fix enabled vertex buffer mask calculation

the mask can't entirely be calculated based on the integer parameters,
as it's possible for some of the "bind" slots to actually be unbinds,
so remove bits as necessary to fix this

also add some debug asserts to ensure I don't break this again for the
tenth time

Fixes: 6dd02a5139a ("zink: stop using util_set_vertex_buffers_mask()")

Reviewed-by: Dave Airlie <airlied@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12871>