drm/i915/selftests: Check preemption rollback of different ring queue depths

Like live_unlite_ring, but instead of simply looking at the impact of
intel_ring_direction(), check that preemption more generally works with
different depths of queued requests in the ring.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200616233733.18050-1-chris@chris-wilson.co.uk