drm/amdkfd: Share the original BO for GTT mapping

If mGPUs is on same IOMMU group, or is ram direct mapped, then mGPUs
can share the original BO for GTT mapping dma address, without creating
new BO from export/import dmabuf.

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>