Fix backlight for ivybridge based laptops (and whatever else comes through
this codepath.)

(1<<30) is documented as BLM_PCH_OVERRIDE_ENABLE, which the current
i915 driver in Linux only sets for broadwell chips.

This fixes the backlight control on the Lenovo X230.

Approved by: kib
Obtained from: Linux 7aa1e7f06d6ea1bce3b27630d50769d13da28b1a