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>

drivers/gpu/drm/drm_crtc_helper.c

@@ -220,6 +220,15 @@ static void __drm_helper_disable_unused_functions(struct drm_device *dev)
  * disconnected connectors. Then it will disable all unused encoders and CRTCs
  * either by calling their disable callback if available or by calling their
  * dpms callback with DRM_MODE_DPMS_OFF.
+ *
+ * NOTE:
+ *
+ * This function is part of the legacy modeset helper library and will cause
+ * major confusion with atomic drivers. This is because atomic helpers guarantee
+ * to never call ->disable() hooks on a disabled function, or ->enable() hooks
+ * on an enabled functions. drm_helper_disable_unused_functions() on the other
+ * hand throws such guarantees into the wind and calls disable hooks
+ * unconditionally on unused functions.
  */
 void drm_helper_disable_unused_functions(struct drm_device *dev)
 {