staging/vboxvideo: Add TODO

Noticed while wondering what vboxvideo is using the ->master_set/drop
hooks for.

v2: Polish grammar a bit (Sam).

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Fabio Rafael da Rosa <fdr@pid42.net>
Cc: Nicholas Mc Guire <der.herr@hofr.at>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190204103114.30772-2-daniel.vetter@ffwll.ch