drivers/intel/gma: store uint8_t brightness values in mailbox3

The _BCM function requires a percentage value. While the
brightness in mailbox3 requires a value in uint8_t. Meaning 255 = 100%.
Previous implementation stored the percentage brightness value resulting
in limiting the brightness to ~40% of it's maximum power.

Only affects brightness control using mailbox3.

Fixes: 6838aaebf9 ("drvs/intel/gma/acpi: Add methods to use MBOX3")
Change-Id: I290b5f5b2a8ee406e39e86d3e0de9997798d890d
Signed-off-by: Alexander Couzens <lynxis@fe80.eu>
Reviewed-on: https://review.coreboot.org/28345
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>

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

@@ -69,7 +69,14 @@
 				Return (Ones)
 			}
 
-			Store (Or (Arg0, 0x80000000), BCLP)
+			/* BCLP requires brightness unsigned 8bit. 255 = 100 % */
+			Store (Divide (Multiply (Arg0, 255), 100), Local1)
+			If (LGreater(Local1, 255)) {
+				Store (255, Local1)
+			}
+			/* set valid bit */
+			Store (Or (Local1, 0x80000000), BCLP)
+
 			/* Request back-light change */
 			Store (0x2, ASLC)
 			/* Trigger IRQ */