iommu/amd: Remove the unnecessary assignment

The assignment of the global variable 'iommu_detected' has been
moved from amd_iommu_init_dma_ops() to amd_iommu_detect(), so
this patch removes the assignment in amd_iommu_init_dma_ops().

Signed-off-by: Adrian Huang <ahuang12@lenovo.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>