Similarly to PCI where msi-map/msi-mask are used to compute the full RID
(aka DID in ITS speak), use the msi-parent as the discovery mechanism,
since there is no way a device can generally express its ID.

However, since switching to a per-device MSI domain model, the domain
passed to its_pmsi_prepare() is the wrong one, and points to the device's
instead of the ITS'. Bad.

Use the parent domain instead, which is the ITS domain.

Fixes: 80b63cc1cc146 ("irqchip/gic-v3-its: Switch platform MSI to MSI parent")
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240718075804.2245733-1-maz@kernel.org