drm/tegra: gem: Don't attach dma-bufs when not needed

The dma-buf import code currently attaches and maps all imported
dma-bufs to the drm device to get their sgt for mapping to the
directly managed IOMMU domain.

In many cases, like for newer chips (Tegra186+), the directly
managed IOMMU domain is, however, not used. Mapping to the drm
device can also cause issues e.g. with swiotlb since it is not
a real device.

To improve the situation, only attach and map imported dma-bufs
when required.

Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240424051335.2872574-2-cyndis@kapsi.fi