mb/purism/librem_skl: Use ACPI backlight controls

Enables ACPI backlight controls.

Change-Id: Iccf50f427b7555ee1a3ef9cc11a89d532789ac54
Signed-off-by: Benjamin Doron <benjamin.doron00@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/40145
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Wim Vervoorn <wvervoorn@eltan.com>
Reviewed-by: Frans Hendriks <fhendriks@eltan.com>
Reviewed-by: Matt DeVillier <matt.devillier@gmail.com>
diff --git a/src/mainboard/purism/librem_skl/variants/librem13v2/devicetree.cb b/src/mainboard/purism/librem_skl/variants/librem13v2/devicetree.cb
index f69c482..6c55af6 100644
--- a/src/mainboard/purism/librem_skl/variants/librem13v2/devicetree.cb
+++ b/src/mainboard/purism/librem_skl/variants/librem13v2/devicetree.cb
@@ -8,6 +8,9 @@
 
 	register "gpu_pch_backlight_pwm_hz"	  = "200"
 
+	# IGD Displays
+	register "gfx" = "GMA_STATIC_DISPLAYS(0)"
+
 	register "deep_s3_enable_ac" = "0"
 	register "deep_s3_enable_dc" = "0"
 	register "deep_s5_enable_ac" = "0"