staging: vboxvideo: make a couple of funcs static

Fix Sparse warnings:
drivers/staging/vboxvideo/vbox_mode.c:309:6: warning:
symbol 'vbox_primary_atomic_disable' was not declared. Should it be static?

drivers/staging/vboxvideo/vbox_mode.c:452:6: warning:
symbol 'vbox_cursor_atomic_disable' was not declared. Should it be static?

Signed-off-by: Craig Kewley <craigkewley@gmail.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>