drm/tegra: gem: Use sg_alloc_table_from_pages()

drm/tegra: gem: Use sg_alloc_table_from_pages()

Instead of manually creating the SG table for a discontiguous buffer,
use the existing sg_alloc_table_from_pages(). Note that this is not safe
to be used with the ARM DMA/IOMMU integration code because that will not
ensure that the whole buffer is mapped contiguously. Depending on the
size of the individual entries the mapping may end up containing holes
to ensure alignment.

However, we only ever use these buffers with explicit IOMMU API usage
and know how to avoid these holes.

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