drivers/generic/gfx: Add null pointer error check

acpi_device_scope() will return NULL if it is unable to find the path
of the parent device. Return early if this is the case to prevent a null
pointer dereference.

Change-Id: I3eff1c1e3477c75c7130b52898de7d59692ba412
Signed-off-by: Jacob Garber <jgarber1@ualberta.ca>
Found-by: Coverity CID 1409672
Reviewed-on: https://review.coreboot.org/c/coreboot/+/38669
Reviewed-by: David Hendricks <david.hendricks@gmail.com>
Reviewed-by: HAOUAS Elyes <ehaouas@noos.fr>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/drivers/generic/gfx/gfx.c

@@ -69,6 +69,9 @@ static void gfx_fill_ssdt_generator(struct device *dev)
 
 	const char *scope = acpi_device_scope(dev);
 
+	if (!scope)
+		return;
+
 	acpigen_write_scope(scope);
 
 	/* Method (_DOD, 0) */