drm: update the documentation of drm_framebuffer_unregister_private

Add obvious description to drm_framebuffer_unregister_private()
to explain it is deprecated.
Signed-off-by: Rongrong Zou <zourongrong@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1477915196-100299-1-git-send-email-zourongrong@gmail.com

drivers/gpu/drm/drm_framebuffer.c

@@ -673,6 +673,11 @@ EXPORT_SYMBOL(drm_framebuffer_lookup);
  * those used for fbdev. Note that the caller must hold a reference of it's own,
  * i.e. the object may not be destroyed through this call (since it'll lead to a
  * locking inversion).
+ *
+ * NOTE: This function is deprecated. For driver-private framebuffers it is not
+ * recommended to embed a framebuffer struct info fbdev struct, instead, a
+ * framebuffer pointer is preferred and drm_framebuffer_unreference() should be
+ * called when the framebuffer is to be cleaned up.
  */
 void drm_framebuffer_unregister_private(struct drm_framebuffer *fb)
 {