drm/i915: Include the last semaphore sync point in the error-state

Should be useful to know what the driver thought the other ring's seqno
was when it last used a semaphore.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>