iommu/amd: Fix irq remapping detection logic

Commit 7fa1c842 "iommu/irq_remapping: Change variable
disable_irq_remap to be static" returns unconditionally success from
the irq remapping prepare callback if the iommu can be initialized.

The change assumed that iommu_go_to_state(IOMMU_ACPI_FINISHED) returns
a failure if irq remapping is not enabled, but thats not the case.

The function returns success when the iommu is initialized to the
point which is required for remapping to work. The actual state of the
irq remapping feature is reflected in the status variable
amd_iommu_irq_remap, which is not considered in the return value.

The fix is simple: If the iommu_go_to_state() returns success,
evaluate the remapping state amd_iommu_irq_remap and reflect it in the
return value.

Fixes: 7fa1c842 iommu/irq_remapping: Change variable disable_irq_remap to be static
Reported-and-tested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Jiang Liu <jiang.liu@linux.intel.com>
Cc: Joerg Roedel <joro@8bytes.org>