drm: Use list_for_each_entry() for bus traversal

Replace list_for_each() + pci_bus_b() with list_for_each_entry().

Signed-off-by: Yijing Wang <[e-mail]>
Signed-off-by: Bjorn Helgaas <[e-mail]>

drivers/gpu/drm/drm_fops.c

···
 			pci_dev_put(pci_dev);
 		}
 		if (!dev->hose) {
-			struct pci_bus *b = pci_bus_b(pci_root_buses.next);
+			struct pci_bus *b = list_entry(pci_root_buses.next,
+				struct pci_bus, node);
 			if (b)
 				dev->hose = b->sysdata;
 		}