drm: Document use-after-free gotcha with private objects

The private objects have a gotcha that could result in a use-after-free,
make sure it's properly documented.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20201204151138.1739736-3-maxime@cerno.tech