i965: allocate a SGVS element when VertexID or InstanceID are read

i965: allocate a SGVS element when VertexID or InstanceID are read

Although on gen8+ platforms we can in theory use 3DSTATE_VF_SGVS
to put these beyond the last vertex element it seems that we still
need to allocate the SVGS element, otherwise we have observed cases
where we end up reading garbage. Specifically, the CTS test mentioned
below was flaky with a fail rate of ~1% on some gen9+ platforms caused
by reading garbage for the gl_InstanceID value. The flakyness goes
away as soon as we start allocating the SVGS element.

v2:
  - Do this for gen8+, not just gen9+, and pull the boolean
    outside the #if block (Jason)

Fixes flaky test:
KHR-GL45.vertex_attrib_64bit.limits_test

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104335
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>