drivers/intel/gma: Don't advertise support for opregion mailbox #2

IGD mailbox #2, Software SCI Interface, is not supported by coreboot
currently, as it requires supporting the Get BIOS Data (GBDA) and
System BIOS Callbacks (SBCB) interfaces. Since coreboot doesn't
support these, don't advertise mailbox #2 support.

This eliminates an error with the Linux display drivers:
"SWSCI request timed out"

TEST=build/boot Linux 6.9, Win11 on starlabs/starlite_adl

Change-Id: I8efcf9c5d384b6e0ce159d65cb1497c2e2e47f42
Signed-off-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/86276
Reviewed-by: Sean Rhodes <sean@starlabs.systems>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jérémy Compostella <jeremy.compostella@intel.com>

Author: MrChromebox

src/drivers/intel/gma/opregion.h

@@ -41,8 +41,7 @@ typedef struct {
 #define IGD_MBOX4	(1 << 3)
 #define IGD_MBOX5	(1 << 4)
 
-#define MAILBOXES_MOBILE  (IGD_MBOX1 | IGD_MBOX2 | IGD_MBOX3 | \
-			   IGD_MBOX4 | IGD_MBOX5)
+#define MAILBOXES_MOBILE  (IGD_MBOX1 | IGD_MBOX3 | IGD_MBOX4 | IGD_MBOX5)
 #define MAILBOXES_DESKTOP (IGD_MBOX2 | IGD_MBOX4)
 
 #define SBIOS_VERSION_SIZE 32