drm/i915: check for kms in dri1 ioctls

Calling these when gem assumes full control of the hw won't end
in anything else than tears. So be a bit more paranoid here.

Just serves as documentation.

v2: Bail out with ENODEV as suggested by Chris Wilson.

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