drm/tegra: gem: Properly pin imported buffers

Buffers that are imported from a DMA-BUF don't have pages allocated with
them. At the same time an SG table for them can't be derived using the
DMA API helpers because the necessary information doesn't exist. However
there's already an SG table that was created during import, so this can
simply be duplicated.

Signed-off-by: Thierry Reding <treding@nvidia.com>