drm/i915: Mark the removal of the i915_request from the sched.link

Keep the rq->fence.flags consistent with the status of the
rq->sched.link, and clear the associated bits when decoupling the link
on retirement (as we may wish to inspect those flags independent of
other state).

Fixes: 32ff621f ("drm/i915/gt: Allow temporary suspension of inflight requests")
References: https://gitlab.freedesktop.org/drm/intel/issues/997


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200122140243.495621-3-chris@chris-wilson.co.uk