drm/doc: Add a warning to drm_dev_is_unplugged

It's probably not what you want, definitely not after Noralf's work to
add drm_dev_enter/exit.

Cc: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190129085643.16357-1-daniel.vetter@ffwll.ch