anv: Rework the descriptor set layout create loop

anv: Rework the descriptor set layout create loop

Previously, we were storing the per-binding create info pointer in the
immutable_samplers field temporarily so that we can switch the order in
which we walk the loop.  However, now that we have multiple arrays of
structs to walk, it makes more sense to store an index of some sort.
Because we want to leave immutable_samplers as NULL for undefined
bindings, we store index + 1 and then subtract one later.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>