mb/google/brya/base/nissa: Add default GMA panel

Enables ACPI brightness controls to be generated, and display
brightness controls to be functional under Windows.

TEST=build/boot Win11 on google/brya (craaskin), verify display
brightness controls present and functional.

Change-Id: I821b912cf52b5b89c5c9d831a5a15566b1b31639
Signed-off-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/83071
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Subrata Banik <subratabanik@google.com>
Reviewed-by: Eric Lai <ericllai@google.com>

src/mainboard/google/brya/variants/baseboard/nissa/devicetree.cb

@@ -185,6 +185,7 @@ chip soc/intel/alderlake
 			    .backlight_off_delay_ms = 200,
 			    .backlight_pwm_hz = 200,
 			}"
+			register "gfx" = "GMA_DEFAULT_PANEL(0)"
 		end
 		device ref dtt on end
 		device ref tcss_xhci on end