PCI/MSI: Move mask and unmask helpers to msi.h

The upcoming support for per device MSI interrupt domains needs to share
some of the inline helpers with the MSI implementation.

Move them to the header file.

Signed-off-by: Ahmed S. Darwish <darwi@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://lore.kernel.org/r/20221111122014.640052354@linutronix.de