drivers/intel/gma/acpi: Replace Decrement() with ASL 2.0 syntax

Replace `Decrement (a)` with `a--`.

Change-Id: I45c3d339652dd457cd4664ed03123eee2d7a5684
Signed-off-by: Felix Singer <felixsinger@posteo.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/60589
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: HAOUAS Elyes <ehaouas@noos.fr>

src/drivers/intel/gma/acpi/common.asl

@@ -40,7 +40,7 @@
 			Store (BRID (XBQC ()), Local0)
 			If (LNotEqual (Local0, 2))
 			{
-				Decrement (Local0)
+				Local0--
 			}
 			XBCM (DerefOf (BRIG[Local0]))
 		}

src/drivers/intel/gma/acpi/configure_brightness_levels.asl

@@ -83,7 +83,7 @@
 						Return (Ones)
 					}
 				}
-				Decrement (Local0)
+				Local0--
 			}
 
 			Return (Ones)