[AGPGART] Make agp=off work again.

This option only worked for the amd64 driver.
On every other driver, the only thing it did was
make it not printk the banner on startup.

drivers/char/agp/backend.c

@@ -243,6 +243,9 @@ int agp_add_bridge(struct agp_bridge_data *bridge)
 {
 	int error;
 
+	if (agp_off)
+		return -ENODEV;
+
 	if (!bridge->dev) {
 		printk (KERN_DEBUG PFX "Erk, registering with no pci_dev!\n");
 		return -EINVAL;