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>
HACK: drm/nouveau: prime: Pin buffer objects to VRAM

This is currently required to work around the lack of proper SMMU
support on Tegra. Ideally buffer objects could always be pinned to GART
and the SMMU will take care of mapping them to a linear I/O virtual
address range for importers.

Signed-off-by: Thierry Reding <treding@nvidia.com>
[HACK] fix video memory mappings

On GK20A video memory is actually system memory. Make sure it is mapped
without compressed storage types, and with the L2 cache disabled.

This is all the more reason for RAM-less devices to report 0 VRAM and
have user-space allocate all their objects in system memory.