iommu/amd: Remove domain search for PCI/MSI

Now that the domain can be retrieved through device::msi_domain the domain
search for PCI_MSI[X] is not longer required. Remove it.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20200826112334.400700807@linutronix.de