drm/i915: Refactor gen8 semaphore offset calculation

We reuse the same calculation into two macros, and I want to add a third
user. Time to refactor.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1460010558-10705-5-git-send-email-chris@chris-wilson.co.uk