staging/vboxvideo: Refactor vbox_update_mode_hints

Refactor vbox_update_mode_hints to no longer use the obsolete
drm_modeset_lock_all() and switch it over to drm_connector_list_iter
instead of directly accessing the list using list_for_each_entry.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190304164724.10210-3-hdegoede@redhat.com