drm/amdgpu: Handle IOMMU enabled case.

drm/amdgpu: Handle IOMMU enabled case.

Problem:
Handle all DMA IOMMU group related dependencies before the
group is removed. Those manifest themself in that when IOMMU
enabled DMA map/unmap is dependent on the presence of IOMMU
group the device belongs to but, this group is released once
the device is removed from PCI topology.

Fix:
Expedite all such unmap operations to pci remove driver callback.

v5: Drop IOMMU notifier and switch to lockless call to ttm_tt_unpopulate
v6: Drop the BO unamp list
v7:
Drop amdgpu_gart_fini
In amdgpu_ih_ring_fini do uncinditional  check (!ih->ring)
to avoid freeing uniniitalized rings.
Call amdgpu_ih_ring_fini unconditionally.
v8: Add deatiled explanation

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210517143851.475058-1-andrey.grodzovsky@amd.com