Comment 0 for bug 2032704

[Impact]
Brightness can't be changed or can only be changed once on some modern
HP laptops.

[Fix]
BIOS folks identified the issue where the ACPI _PS0 method isn't called
for the panel device. On Windows the method is being invoked.

This is due to missing _PSC, but since Windows is the de facto spec, we
have to perform the same to make the device functional.

[Test]
Once the fix is applied, the brightness can be changed smoothly.

[Where problems could occur]
According to BIOS folks, Windows also invokes _PS3 for sleep and
shutdown, we need to tackle that in the future.
Right now not calling _PS3 has no ill-effect, per BIOS folks.