mb/system76: ADL: Add gfx register for GMA ACPI

Add gfx register to System76 ADL boards so GMA ACPI data is generated.
Fixes backlight controls on Windows 10 and Linux 6.1.

Change-Id: I356e09350ee0f1412409509a2b1695642ae210b3
Signed-off-by: Tim Crawford <tcrawford@system76.com>

src/mainboard/system76/adl-p/devicetree.cb

@@ -32,6 +32,8 @@ chip soc/intel/alderlake
 				[DDI_PORT_A] = DDI_ENABLE_HPD,
 				[DDI_PORT_B] = DDI_ENABLE_HPD | DDI_ENABLE_DDC,
 			}"
+
+			register "gfx" = "GMA_DEFAULT_PANEL(0)"
 		end
 		device ref tbt_pcie_rp0 on end
 		device ref shared_sram on end

src/mainboard/system76/gaze17/variants/3050/overridetree.cb

@@ -9,6 +9,8 @@ chip soc/intel/alderlake
 				[DDI_PORT_A] = DDI_ENABLE_HPD,
 				[DDI_PORT_B] = DDI_ENABLE_HPD | DDI_ENABLE_DDC,
 			}"
+
+			register "gfx" = "GMA_DEFAULT_PANEL(0)"
 		end
 		device ref pcie4_0 on
 			# PCIe PEG0 x4, Clock 0 (SSD2)

src/mainboard/system76/gaze17/variants/3060/overridetree.cb

@@ -6,6 +6,8 @@ chip soc/intel/alderlake
 			# DDIA is eDP
 			register "ddi_portA_config" = "1"
 			register "ddi_ports_config[DDI_PORT_A]" = "DDI_ENABLE_HPD"
+
+			register "gfx" = "GMA_DEFAULT_PANEL(0)"
 		end
 		device ref pcie4_0 on
 			# PCIe PEG0 x4, Clock 0 (SSD2)