PM / Intel IOMMU: Fix init_iommu_pm_ops() for CONFIG_PM unset

If CONFIG_PM is not set, init_iommu_pm_ops() introduced by commit
134fac3f (PCI / Intel IOMMU: Use
syscore_ops instead of sysdev class and sysdev) is not defined
appropriately.  Fix this issue.

Reported-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>