drm/sysfs: Annote lockless show functions with READ_ONCE

For documentation and paranoia.

Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1459331120-27864-4-git-send-email-daniel.vetter@ffwll.ch