platform/x86: thinkpad_acpi: Get privacy-screen / lcdshadow ACPI handles only once

Get the privacy-screen / lcdshadow ACPI handles once and cache them,
instead of retrieving them every time we need them.

Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Mark Pearson <markpearson@lenovo.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211005202322.700909-8-hdegoede@redhat.com