drm/i915: Dequeue execlists on a new request if any port is available

If the second ELSP port is available, schedule the execlists tasklet to
see if the new request can occupy it.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170124110009.28947-7-chris@chris-wilson.co.uk