drm/i915/hangcheck: Track context changes

drm/i915/hangcheck: Track context changes

Given sufficient preemption, we may see a busy system that doesn't
advance seqno while performing work across multiple contexts, and given
sufficient pathology not even notice a change in ACTHD. What does change
between the preempting contexts is their RING, so take note of that and
treat a change in the ring address as being an indication of forward
progress.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190501114541.10077-1-chris@chris-wilson.co.uk