mb/system76: Set gfx register

Fixes brightness controls on Windows 10.

Change-Id: I33ac1b5a17c95dbb1b166c38fcd639cdac439724
Signed-off-by: Tim Crawford <tcrawford@system76.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/67636
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jeremy Soller <jeremy@system76.com>

src/mainboard/system76/gaze16/devicetree.cb

@@ -95,6 +95,8 @@ chip soc/intel/tigerlake
 			register "DdiPortBConfig" = "DDI_PORT_CFG_NO_LFP"
 			register "DdiPortBHpd" = "1"
 			register "DdiPortBDdc" = "1"
+
+			register "gfx" = "GMA_DEFAULT_PANEL(0)"
 		end
 		device ref dptf on end
 		device ref gna on end

src/mainboard/system76/lemp9/devicetree.cb

@@ -61,7 +61,9 @@ chip soc/intel/cannonlake
 	device domain 0 on
 		subsystemid 0x1558 0x1401 inherit
 		device pci 00.0 on  end # Host Bridge
-		device pci 02.0 on  end # Integrated Graphics Device
+		device pci 02.0 on      # Integrated Graphics Device
+			register "gfx" = "GMA_DEFAULT_PANEL(0)"
+		end
 		device pci 04.0 on      # SA Thermal device
 			register "Device4Enable" = "1"
 		end

src/mainboard/system76/oryp8/devicetree.cb

@@ -108,6 +108,8 @@ chip soc/intel/tigerlake
 			register "DdiPortAConfig" = "DDI_PORT_CFG_EDP"
 			register "DdiPortAHpd" = "1"
 			register "DdiPortADdc" = "0"
+
+			register "gfx" = "GMA_DEFAULT_PANEL(0)"
 		end
 		device ref dptf on end
 		device ref peg0 on