iommu/amd: Use msi_msg shadow structs

Get rid of the macro mess and use the shadow structs for the x86 specific
MSI message format. Convert the intcapxt setup to use named bitfields as
well while touching it anyway.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20201024213535.443185-15-dwmw2@infradead.org