staging: vboxvideo: Update driver to use drm_dev_register.

The use of load and unload hooks is deprecated. DRM drivers should
use drm_dev_alloc|drm_dev_init and drm_dev_register for initialization
and publishing.

Signed-off-by: Fabio Rafael da Rosa <fdr@pid42.net>
Reviewed-by: Nicholas Mc Guire <der.herr@hofr.at>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>