drm/i915: Enable semaphores for legacy submission on gen8

We have sufficient evidence from igt to support that semaphores are in
a working state. Enabling semaphores now for legacy provides a better
comparison of execlists against legacy ring submission.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1461928823-10298-6-git-send-email-chris@chris-wilson.co.uk