drm/i915/selftests: Run MI_BB perf selftests on SNB

SNB does have the RING_TIMESTAMP register on the RCS engine.
Run the MI_BB perf tests on it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221031135703.14670-5-ville.syrjala@linux.intel.com


Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>