i965: GS: Use the VUE map to compute URB size.

i965: GS: Use the VUE map to compute URB size.

The previous computation had two bugs: (a) it used a formula based on
Gen5 for Gen6 and Gen7 as well. (b) it failed to account for the fact
that PSIZ is stored in the VUE header.  Fortunately, both bugs caused
it to compute a URB size that was too large, which was benign.  This
patch computes the URB size directly from the VUE map, so it gets the
result correct in all circumstances.

Reviewed-by: Eric Anholt <eric@anholt.net>