drm/tegra: Support DMA API for display controllers

If a display controller is not attached to an explicit IOMMU domain,
which usually means that it's connected to an IOMMU domain controlled by
the DMA API, make sure to map the framebuffer to the display controller
address space. This allows us to transparently handle setups where the
display controller is attached to an IOMMU or setups where it isn't. It
also allows the driver to work with a DMA API that is backed by an
IOMMU.

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