If GISA is available, we do not have to kick CPUs out of SIE to deliver
interrupts. The hardware can deliver such interrupts while running.

Cc: Michael Mueller <mimu@linux.vnet.ibm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>