iommu/amd: Check feature support bit before accessing MSI capability registers

The IOMMU MMIO access to MSI capability registers is available only if
the EFR[MsiCapMmioSup] is set. Current implementation assumes this bit
is set if the EFR[XtSup] is set, which might not be the case.

Fix by checking the EFR[MsiCapMmioSup] before accessing the MSI address
low/high and MSI data registers via the MMIO.

Fixes: 66929812 ('iommu/amd: Add support for X2APIC IOMMU interrupts')
Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>