mb/google/fatcat/var/fatcat: Enable iGPU display using FW_CONFIG

This change enables the integrated GPU (iGPU) display on the Fatcat
board based on the FW_CONFIG setting (specifically the DISPLAY bit).

By conditionally probing the display based on FW_CONFIG, the iGPU
is dynamically enabled or disabled according to the SKU configuration.

TEST=Verified display functionality on Fatcat with the iGPU:

> cbi set 6 0x58A814 4 (DISPLAY_ABSENT):
 - lspci does not list the iGPU.
 - No display output, but the device boots to the OS (verified via
   console).

> cbi set 6 0x5CA814 4 (DISPLAY_PRESENT):
 - lspci lists the iGPU.
 - Display output is functional, showing firmware and OS UI.

Change-Id: I5762adf5ec8a86a00c16544670cb2f998055bd35
Signed-off-by: Subrata Banik <subratabanik@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/84877
Reviewed-by: Jayvik Desai <jayvik@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Pranava Y N <pranavayn@google.com>
Reviewed-by: Eric Lai <ericllai@google.com>

src/mainboard/google/fatcat/variants/fatcat/overridetree.cb

@@ -136,6 +136,10 @@ chip soc/intel/pantherlake
 	}"
 
 	device domain 0 on
+		device ref igpu on
+			probe DISPLAY DISPLAY_PRESENT
+		end
+
 		device ref ipu on
 			chip drivers/intel/mipi_camera
 				register "acpi_uid" = "0x50000"