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: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>