drm/i915: Trim the retired request queue after submitting

If we submit a request and see that the previous request on this
timeline was already signaled, we first do not need to add the
dependency tracker for that completed request and secondly we know that
we there is then a large backlog in retiring requests affecting this
timeline. Given that we just submitted more work to the HW, now would be
a good time to catch up on those retirements.

v2: Try to sum up the compromises involved in flushing the retirement
queue after submission.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180207084350.3929-1-chris@chris-wilson.co.uk