drm/tegra: dc: Inherit DMA mask

Inherit the DMA mask from host1x (on Tegra210 and earlier) or the
display hub (on Tegra186 and later). This is necessary in order to
properly map buffers without SMMU support and use the maximum IOVA
space available with SMMU support.

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