drm/i915: Ditch UMS config option

drm/i915: Ditch UMS config option

Let's march ahead with the deprecation plan laid out in

commit b30324adaf8d2e5950a602bde63030d15a61826f
Author: Daniel Vetter <[email protected]>
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 <[email protected]>
Cc: David Herrmann <[email protected]>
Suggested-by: David Herrmann <[email protected]>
Acked-by: Dave Airlie <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>