drm/i915: add schedule out notification of preempted but completed request

There is one corner case missing schedule out notification of the preempted
request. The preempted request is just completed when preemption happen,
then it will be canceled and won't be resubmitted later, GVT-g will lost
the schedule out notification.

Here add schedule out notification if found the preempted request has been
completed.

v2:
- refine description, add completed check and notification in
  execlists_cancel_port_requests. (Chris)

v3:
- use ternary confitional, remove local variable. (Tvrtko)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Weinan Li <weinan.z.li@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1520302557-25079-1-git-send-email-weinan.z.li@intel.com