drm/i915: Emit dma-fence (and execlists submit) first from signaler

When introduced, I thought that reducing client latency from the
signaler was the priority. Since its inception the signaler has become
responsible for keeping the execlists full, via the dma-fence. As this
is very important to minimise overall execution time, signal the
dma-fence first and then signal any waiting clients.

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/20170124110009.28947-8-chris@chris-wilson.co.uk