drm2 regression: backlight adjustment on ivybridge no longer works

[Adrian Chadd]

ugh, so because i can't sleep, I read the Linux i915 history for
backlight changes.

Turns out if I disable bit 30 in ivb_pch_pwm_override() to register
BLC_PWM_PCH_CTL1, the acpi video setting works. The keyboard settings
still don't, but I'll worry about that later.

Bit 30 in the linux drm code is:

#define   BLM_PCH_OVERRIDE_ENABLE               (1 << 30)

The linux dri code doesn't set that for ivybridge - only for broadwell.

(also, the linux driver has POSTING_READ() which I'm guessing is to
ensure things made it out to the device and have been flushed before
doing another IO operation.)

I have my (via sysctl) panel backlight control working again.



-adrian