drm/amdgpu: Add multi-GPU DMA mapping helpers

Add BO-type specific helpers functions to DMA-map and unmap
kfd_mem_attachments. Implement this functionality for userptrs by creating
one SG BO per GPU and filling it with a DMA mapping of the pages from the
original mem->bo.

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>