drivers/intel/gma: Include gfx.asl by default for all platforms...

which select INTEL_GMA_ACPI. Rework brightness level includes and
platform-level asl files to avoid duplicate device definition for GFX0.

Include gfx.asl for Skylake/Kabylake, since all other soc/intel/common
platforms already do. Adjust mb/51nb/x210 to prevent device redefinition.

Some OSes (e.g. Windows, MacOS) require/prefer the ACPI device for
the IGD to exist, even if ACPI brightness controls are not utilized.
This change adds a GFX0 ACPI device for all boards whose platforms
select INTEL_GMA_ACPI without requiring non-functional brightness
controls to be added at the board level.

Change-Id: Ie71bd5fc7acd926b7ce7da17fbc108670fd453e0
Signed-off-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/48862
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Michael Niewöhner <foss@mniewoehner.de>

src/soc/intel/baytrail/acpi/southcluster.asl

@@ -256,3 +256,6 @@ Scope (\_SB.PCI0)
 	// LPE Device
 	#include "lpe.asl"
 }
+
+/* Integrated graphics 0:2.0 */
+#include <drivers/intel/gma/acpi/gfx.asl>