PCI/MSI: Handle the NOMASK flag correctly for all PCI/MSI backends

The conversion of the XEN specific global variable pci_msi_ignore_mask to a
MSI domain flag, missed the facts that:

    1) Legacy architectures do not provide a interrupt domain
    2) Parent MSI domains do not necessarily have a domain info attached
   
Both cases result in an unconditional NULL pointer dereference. This was
unfortunatly missed in review and testing revealed it late.

Cure this by using the existing pci_msi_domain_supports() helper, which
handles all possible cases correctly.

Fixes: c3164d2e ("PCI/MSI: Convert pci_msi_ignore_mask to per MSI domain flag")
Reported-by: Daniel Gomez <da.gomez@kernel.org>
Reported-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Juergen Gross <jgross@suse.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Borislav Petkov <bp@alien8.de>
Tested-by: Daniel Gomez <da.gomez@kernel.org>
Link: https://lore.kernel.org/all/87iknwyp2o.ffs@tglx
Closes: https://lore.kernel.org/all/qn7fzggcj6qe6r6gdbwcz23pzdz2jx64aldccmsuheabhmjgrt@tawf5nfwuvw7