mb/system76: Set gfx register

`gfx` got dropped during some rebase. Add them back.

Fixes brightness controls on Windows 10.

Change-Id: Ifd2553e3929962598185cc553c480dcb0087af5c
Signed-off-by: Tim Crawford <tcrawford@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

@@ -104,6 +104,8 @@ 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