drm/amdgpu: Reduce SG bo memory usage for mGPUs

For userptr bo, if adev is not in IOMMU isolation mode, RAM direct map
to GPU, multiple GPUs use same system memory dma mapping address, they
can share the original mem->bo in attachment to reduce dma address array
memory usage.

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>