staging/vboxvideo: Drop initial_mode_queried workaround

Drop the initial_mode_queried workaround for kms clients which do not
support hotplug, all kms clients should be able to deal with hotplug.

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
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/20190304164724.10210-2-hdegoede@redhat.com