[AGPGART] Add SIS 648/746 work around.

These chipsets hang for no apparent reason unless we pause for a while
after changing mode. No explanation from SiS, so this ugly hack is the
best we can hope for.

drivers/char/agp/generic.c

@@ -524,6 +524,14 @@ void agp_device_command(u32 command, int agp_v3)
 		printk(KERN_INFO PFX "Putting AGP V%d device at %s into %dx mode\n",
 				agp_v3 ? 3 : 2, pci_name(device), mode);
 		pci_write_config_dword(device, agp + PCI_AGP_COMMAND, command);
+
+		/*
+		 * Work around SIS648/746 instability by delaying a bit.
+		 * This isn't a particularly nice solution, but in absense of
+		 * info from SiS, it's the best we can do.
+		 */
+		set_current_state(TASK_INTERRUPTIBLE);
+		schedule_timeout((HZ+99)/100); /* >= 1/100th of a second */
 	}
 }
 EXPORT_SYMBOL(agp_device_command);