drm: fix end of loop test

"agpmem" is never NULL here because it is the list cursor of a
list_for_each_entry() list.
Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_vm.c

@@ -138,7 +138,7 @@ static int drm_do_vm_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
 				break;
 		}
 
-		if (!agpmem)
+		if (&agpmem->head == &dev->agp->memory)
 			goto vm_fault_error;
 
 		/*