drm/crtc-helper: Add caveat to disable_unused_functions doc

This shouldn't be used by atomic drivers any more, it confuses the
state tracking.

Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1452695476-31147-1-git-send-email-daniel.vetter@ffwll.ch


Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>