gm45/gma.c: use correct id string for fake VBT

The correct id string for gm45 is "$VBT CANTIGA        ".
This can be found in the gm45 option rom:
"strings vbios.bin | grep VBT".

Change-Id: Icd67a87dac774b4b3c211511c784c4fb4e2ea97c
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/16551
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>

src/northbridge/intel/gm45/gma.c

@@ -741,7 +741,7 @@ static void gma_func0_init(struct device *dev)
 
 		/* Linux relies on VBT for panel info.  */
 		generate_fake_intel_oprom(&conf->gfx, dev,
-					  "$VBT IRONLAKE-MOBILE");
+					"$VBT CANTIGA        ");
 	}
 }