drm/amdgpu: Add userptr bo support for mGPUs when iommu is on

For userptr bo with iommu on, multiple GPUs use same system
memory dma mapping address when both adev and bo_adev are in
identity mode or in the same iommu group.

If RAM direct map to one GPU, other GPUs can share the original
BO in order to reduce dma address array usage when RAM can
direct map to these GPUs. However, we should explicit check
whether RAM can direct map to all these GPUs.

This patch fixes a potential issue that where RAM is
direct mapped on some but not all GPUs.

v2:
  1. Update comment
  2. Add helper function reuse_dmamap

Signed-off-by: Shane Xiao <shane.xiao@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>