PCI/P2PDMA: Ignore root complex whitelist when an IOMMU is present

Presently, there is no path to DMA map P2PDMA memory, so if a TLP targeting
this memory hits the root complex and an IOMMU is present, the IOMMU will
reject the transaction, even if the RC would support P2PDMA.

So until the kernel knows to map these DMA addresses in the IOMMU, we
should not enable the whitelist when an IOMMU is present.

Link: https://lore.kernel.org/linux-pci/20190522201252.2997-1-logang@deltatee.com/
Fixes: 0f97da83

 ("PCI/P2PDMA: Allow P2P DMA between any devices under AMD ZEN Root Complex")
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Christoph Hellwig <hch@lst.de>