agp/intel-gtt: don't require the agp bridge on setup

We only need it to fake the agp interface and don't actually
use it in the driver anywhere. Hence conditionalize that.

This is just a prep patch to eventually disable the fake agp
driver on gen6+.
Reviewed-by: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/char/agp/intel-gtt.c

@@ -1539,9 +1539,11 @@ int intel_gmch_probe(struct pci_dev *pdev,
 	if (!intel_private.driver)
 		return 0;
 
+	if (bridge) {
 		bridge->driver = &intel_fake_agp_driver;
 		bridge->dev_private_data = &intel_private;
 		bridge->dev = pdev;
+	}
 
 	intel_private.bridge_dev = pci_dev_get(pdev);