platform/chrome: chromeos_laptop - parse DMI IRQ data once

Instead of trying to parse DMI IRQ data every time we try to instantiate a
device, let's do it once, when we identify the device we are working with.
This allows us to mark chromeos_laptop_get_irq_from_dmi() as __init and
discard it once module is initialized.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Benson Leung <bleung@chromium.org>