drm/i915: Deprecate UMS harder

Progess according to the deprecation plan laid out in

commit b30324ad
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Wed Nov 13 22:11:25 2013 +0100

    drm/i915: Deprecated UMS support

and disable UMS for 3.16. Note that it has been over 5 years since the
last UMS-supporting piece of userspace was released.

Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>