drm/i915: Reorder i915_add_request to separate the phases better

Let's avoid mixing sealing the hardware commands for the request and
adding the request to the software tracking.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160909131201.16673-15-chris@chris-wilson.co.uk