fb/ramgk20a: use mm_nodes

This is how VRAM objects are defined on other GPUs, and this saves us
from caveats when mapping large pages.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
fb/ramgk20a: support IOMMU if present

Tegra SoCs come with an IOMMU that can make physically non-contiguous
regions of memory appear contiguous to the GPU. This patch adds support
for this unit if present. This greatly optimizes memory usage and allows
GK20A to perform without CMA.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>