When a request is preempted, it is unsubmitted from the HW queue and
removed from the active list of breadcrumbs. In the process, this
however triggers the signaler and it may see the clear rbtree with the
old, and still valid, seqno, or it may match the cleared seqno with the
now zero rq->global_seqno. This confuses the signaler into action and
signaling the fence.

Fixes: d6a2289d ("drm/i915: Remove the preempted request from the execution queue")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: <stable@vger.kernel.org> # v4.12+
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180206094633.30181-1-chris@chris-wilson.co.uk