drivers/intel/gma: Guard legacy brightness fallback

When BOX3.XBCM fails we currently fall back to LEGA.XBCM, which writes
directly to the IGD PWM registers. During S3 resume those registers are
still reset by the graphics driver, so AML stores a zero duty cycle and
the panel stays dark. This leads to having some other event needed to
wake the panel (i.e. key press).

Only invoke the legacy path after BCLM is initialized, matching when the
driver has reprogrammed the PWM registers and preventing firmware from
touching them while the driver is still restoring them.

Test=Enter and exit S3 on starbook_mtl, verify that the display turns on and stays on, instead of on -> off -> on.

Change-Id: I664d296372feef9de5c4f57428422328c4e33110
Signed-off-by: Sean Rhodes <sean@starlabs.systems>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/89985
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Matt DeVillier <matt.devillier@gmail.com>

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

@@ -149,7 +149,14 @@
 	{
 		If (^BOX3.XBCM (Arg0) == Ones)
 		{
-			^LEGA.XBCM (Arg0)
+			/*
+			 * Only touch the legacy PWM registers after the graphics driver
+			 * reprograms them during boot/resume (BCLM stays zero until then).
+			 */
+			If (BCLM != 0)
+			{
+				^LEGA.XBCM (Arg0)
+			}
 		}
 	}