drivers/intel/opregion.c: Also set vbt_size if size is 0

Make sure size vbt_size is initialized. GCC LTO warns about this.

Change-Id: I4fcc6c02f898640e9b40d769e1165a4a0fb0fdf2
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/84041
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nicholas Sudsgaard <devel+coreboot@nsudsgaard.com>

src/drivers/intel/gma/opregion.c

@@ -49,7 +49,7 @@ err:
 	size = 0;
 
 out:
-	if (vbt_size && size)
+	if (vbt_size)
 		*vbt_size = size;
 	return data;
 }