When virtfn is used, we should use physfn to find correct drhd

-v2: add pci_physfn() Suggested by Roland Dreier <rdreier@cisco.com>
     do can remove ifdef in dmar.c
-v3: Chris pointed out we need that for dma_find_matched_atsr_unit too
     also change dmar_pci_device_match() static
Signed-off-by: NYinghai Lu <yinghai@kernel.org>
Acked-by: NRoland Dreier <rdreier@cisco.com>
Acked-by: NChris Wright <chrisw@sous-sol.org>
Acked-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NDavid Woodhouse <David.Woodhouse@intel.com>