intel-iommu: remove some unused struct intel_iommu fields

The seg, saved_msg and sysdev fields appear to be unused since
before the code was first merged.

linux/msi.h is not needed in linux/intel-iommu.h anymore since
there is no longer a reference to struct msi_msg. The MSI code
in drivers/pci/intel-iommu.c still has linux/msi.h included
via linux/dmar.h.

linux/sysdev.h isn't needed because there is no reference to
struct sys_device.

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>