[Intel-gfx] VLV backlight bits

[Jesse Barnes]

Updated these to include Jani's review feedback.  Ultimately though, we
need to expand this to properly handle dual-panel support on BYT, or
figure out which is the correct connector in the OpRegion callback code.
I still have an open request from the VBIOS team about that.

And Ville is right that these bits need to be pushed into the connector.
What I'd like to do is make them virtual functions of the connector,
with the VLV, gen4, and PCH bits split out.  We can save/restore the
state there too from modeset_init_hw, and maybe drop some bits now that
we don't reset the display by default.

But all of that is for follow on patches.

Thanks,
Jesse