drm/i915/selftests: Skip RPS tests on Ironlake (only IPS)

Since Ironlake uses intel_ips.ko for its dynamic frequency adjustment,
we do not have direct control over the frequency management so such
tests are defunct. Similarly, we can't check the gen6+ RPS registers on
Ironlake.

Hopefully this catches all the invalid tests now that Ironlake has
rejoined the dynamic GPU frequency club. There is an opportunity for the
reader to add tests to exercise MEMINTRSTS and co.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201022210814.23004-1-chris@chris-wilson.co.uk