gallivm/nir: fix vulkan vertex inputs

the tgsi file max is used to determine the number of input slots,
the old code was pretty bogus for the lavapipe cases (it worked
by accident).

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7416>