[PowerPC] Fix NUMA checking for powernv

At this point in startup, vm_ndomains has not been initialized. Switch
to checking kenv instead.

Fixes incorrect NUMA information being set on multi-domain systems like
Talos II.

Submitted by:	jhibbits
MFC after:	2 weeks

sys/powerpc/powernv/platform_powernv.c

@@ -532,7 +532,9 @@ powernv_node_numa_domain(platform_t platform, phandle_t node)
 #ifndef NUMA
 	return (0);
 #endif
-	if (vm_ndomains == 1)
+	i = 0;
+	TUNABLE_INT_FETCH("vm.numa.disabled", &i);
+	if (i)
 		return (0);
 
 	res = OF_getencprop(node, "ibm,associativity",