iommu/amd: Unexport get_dev_data()

This function is internal to the AMD IOMMU driver and only exported
because the amd_iommu_v2 modules calls it. But the reason it is called
from there could better be handled by amd_iommu_is_attach_deferred().
So unexport get_dev_data() and use amd_iommu_is_attach_deferred()
instead.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Reviewed-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Link: https://lore.kernel.org/r/20200527115313.7426-3-joro@8bytes.org