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