There were two instances of code to control the panel backlight and
neither handled the complete set of device variations.

Fixes:

  Bug 29716 - [GM965] Regression: Backlight resets to minimum when changing resolution
  https://bugs.freedesktop.org/show_bug.cgi?id=29716

And a bug on one of my PineView boxes which overflowed the backlight
value.

Incorporates part of a similar patch by Matthew Garrett that exposes a
native Intel backlight controller.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>

v2: Hook in DP paths to keep FULLSCREEN panel fitting on eDP.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NZhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: NEric Anholt <eric@anholt.net>