drm/i915/execlists: Avoid kicking the submission too early for rescheduling

drm/i915/execlists: Avoid kicking the submission too early for rescheduling

If the request is still waiting on external fences, it has not yet been
submitted to the HW queue and so we can forgo kicking the submission
tasklet when re-evaluating its priority.

This should have no impact other than reducing the number of tasklet
wakeups under signal heavy workloads (e.g. switching between engines).

v2: Use prebaked container_of()

References: f6322eddaff7 ("drm/i915/preemption: Allow preemption between submission ports")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180326115044.2505-2-chris@chris-wilson.co.uk
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>