drm/tegra: Add Tegra DRM allocation API

Add a new IO virtual memory allocation API to allow clients to
allocate non-GEM memory in the Tegra DRM IOMMU domain. This is
required e.g. for loading client firmware when clients are attached
to the IOMMU domain.

The allocator allocates contiguous physical pages that are then
mapped contiguously to the IOMMU domain using the iova_domain
library provided by the kernel. Contiguous physical pages are
used so that the same allocator works also when IOMMU support
is disabled and therefore devices access physical memory directly.

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