drm/i915: Ditch UMS config option

Let's march ahead with 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

Thus far no regression report yet, so the transparent fallback plan
seems to pan out.

Cc: Dave Airlie <airlied@gmail.com>
Cc: David Herrmann <dh.herrmann@gmail.com>
Suggested-by: David Herrmann <dh.herrmann@gmail.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>