drm/vboxvideo: Stop using drm_device->dev_private

We use the baseclass pattern here, so lets to the proper (and more
typesafe) upcasting.

Acked-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200415074034.175360-5-daniel.vetter@ffwll.ch