drm/i915: Tweak scheduler's kick_submission()

Skip useless priority bumping on adding a new dependency by making sure
that we do update the priority if we would have rescheduled the active
cotnext.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200310115947.6482-1-chris@chris-wilson.co.uk