mb/google/glados: Use ACPI brightness controls

Enables ACPI backlight controls under Windows.

Test: build/boot chell variant, verify screen backlight
controls available and functional under Windows 10.

Change-Id: Ida0102ac828254ae195528a8f1de078abed4a491
Signed-off-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/40121
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/mainboard/google/glados/devicetree.cb

@@ -1,5 +1,8 @@
 chip soc/intel/skylake
 
+	# IGD Displays
+	register "gfx" = "GMA_STATIC_DISPLAYS(0)"
+
 	register "gpu_pp_up_delay_ms"		  = "200"
 	register "gpu_pp_down_delay_ms"		  = " 50"
 	register "gpu_pp_cycle_delay_ms"	  = "500"

src/mainboard/google/glados/dsdt.asl

@@ -35,6 +35,7 @@ DefinitionBlock(
 		{
 			#include <soc/intel/skylake/acpi/systemagent.asl>
 			#include <soc/intel/skylake/acpi/pch.asl>
+			#include <drivers/intel/gma/acpi/default_brightness_levels.asl>
 		}
 
 		// Dynamic Platform Thermal Framework