i965/gen7: Make primitives_written counting work.

i965/gen7: Make primitives_written counting work.

The code was relying on gs.prog_data's copy of the
number-of-verts-per-prim, which segfaulted on gen7 since it doesn't
make a GS program.  We can easily calculate that value right here.

v2: Fix svbi_0_starting_index regression.

Reviewed-by: Paul Berry <stereotype441@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>