drm/amdgpu: DMA map/unmap when updating GPU mappings

DMA map kfd_mem_attachments in update_gpuvm_pte. This function is called
with the BO and page tables reserved, so we can safely update the DMA
mapping.

DMA unmap when a BO is unmapped from a GPU and before updating mappings
in restore workers.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Oak Zeng <Oak.Zeng@amd.com>
Acked-by: Ramesh Errabolu <Ramesh.Errabolu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>