drm/tegra: Support kernel mappings with IOMMU

host1x command buffer patching requires that the buffer object can be
mapped into kernel address space. However, the recent addition of IOMMU
support did not account for this requirement. Therefore host1x engines
cannot be used if IOMMU is enabled.

This patch implements kmap, kunmap, mmap and munmap functions to host1x
buffer objects.

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