drm/tegra: gem: Take into account IOMMU aperture

The IOMMU may not always be able to address 2 GiB of memory. On Tegra20,
the GART supports 32 MiB starting at 0x58000000. Also the aperture on
Tegra30 and later is in fact the full 4 GiB, rather than just 2 GiB as
currently assumed.

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