MFC 270516:
i915 driver - enable opregion handle; program CADL.

add opregion handling for drm2 - which exposes some ACPI video configuration
pieces that some Lenovo laptop models use to flesh out which video device
to speak to. This enables the brightness control in ACPI to work these models.

The CADL bits are also important - it's used to figure out which ACPI
events to hook the brightness buttons into. It doesn't yet seem to work
for me, but it does for the OP.

PR: 190186, 198551
Submitted by: Henry Hu <henry.hu.sh@gmail.com>