iommu/vt-d, trivial: check suitable flag in function detect_intel_iommu()

Flag irq_remapping_enabled is only set by intel_enable_irq_remapping(),
which is called after detect_intel_iommu(). So moving pr_info() from
detect_intel_iommu() to intel_enable_irq_remapping(), which also
slightly simplifies implementation.

Reviewed-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Signed-off-by: Joerg Roedel <joro@8bytes.org>