drivers-platform-x86: use acpi_video_dmi_promote_vendor()

Instead of using directly acpi_video_unregister(), use
acpi_video_dmi_promote_vendor() (and make it call
acpi_video_unregister() if needed)

Signed-off-by: Corentin Chary <corentin.chary@gmail.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>