drm/i915/scheduler: Signal the arrival of a new request

The start of the scheduler, add a hook into request submission for the
scheduler to see the arrival of new requests and prepare its runqueues.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161114204105.29171-6-chris@chris-wilson.co.uk