amd iommu: use base platform MSI implementation

amd iommu: use base platform MSI implementation

Given that here, other than for VT-d, the MSI interface gets surfaced
through a normal PCI device, the code should use as much as possible of
the "normal" MSI support code.

Further, the code can (and should) follow the "normal" MSI code in
distinguishing the maskable and non-maskable cases at the IRQ
controller level rather than checking the respective flag in the
individual actors.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Wang <wei.wang2@amd.com>
Acked-by: Keir Fraser <keir@xen.org>