i965: Fix instanced geometry shaders on Gen8+.

i965: Fix instanced geometry shaders on Gen8+.

Jordan added this in commit 741782b5948bb3d01d699f062a37513c2e73b076 for
Gen7 platforms.  I missed this when adding the Broadwell code.

Fixes Piglit's spec/arb_gpu_shader5/invocation-id-{basic,in-separate-gs}
with MESA_EXTENSION_OVERRIDE=GL_ARB_gpu_shader5 set.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>
Cc: mesa-stable@lists.freedesktop.org