x86, UV: Initialize BAU MMRs only on hubs with cpus

Remove the initialization of MMRs
UVH_LB_BAU_SB_ACTIVATION_CONTROL and UVH_BAU_DATA_BROADCAST on
UV hubs that have no active cpus. Such initialization on hubs
with no active cpus would result in a kernel page fault.

This is not of real high priority, because we don't have any
such systems (with UV hubs that have no active cpus).  But they
will be coming.

Signed-off-by: Cliff Wickman <cpw@sgi.com>
LKML-Reference: <E1OZmZN-0006cW-RC@eag09.americas.sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>