drm/i915: Only queue requests during execlists submission

Leave the more complicated request dequeueing to the tasklet and instead
just kick start the tasklet if we detect we are adding the first
request.

v2: Play around with list operators until we agree upon something

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160909131201.16673-2-chris@chris-wilson.co.uk