iommu/amd: Remove redundant check in irq_remapping_select()

The amd_iommu_irq_remap flag is already validated during irq remapping
domain creation (before calling amd_iommu_create_irq_domain()). The
duplicate check in irq_remapping_select() is unnecessary and can be
removed.

Additionally, mark amd_iommu_irq_remap as static.

No functional changes.

Signed-off-by: Vasant Hegde <vasant.hegde@amd.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>