drm/i915: Only attempt to pass the first request to execlists

Only the first request added to the execlist queue can be submitted. If
this request is not the first request on the queue, it means that there
are already higher priority requests waiting upon the tasklet and
kicking it will make no difference.

This is more relevant for a later patch, where we more eagerly try and
kick the tasklet to handle the submission of new requests.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170124110009.28947-6-chris@chris-wilson.co.uk


Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>