i965: Split the gen6 GS binding table to a separate table.

i965: Split the gen6 GS binding table to a separate table.

Improves VS state change microbenchmark performance by 7.08729% +/-
1.22289% (n=10) on gen7, because we don't upload the 64 dwords of
unused binding table any more.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>