drm/nouveau: use alternate memory type for system-memory buffers with kind != 0

Fixes bug on Tegra where we'd strip kind information from system memory
(ie. all) buffers, resulting in misrendering.

Behaviour on dGPU should be unchanged.

Reported-by: Thierry Reding <treding@nvidia.com>
Fixes: d7722134 ("drm/nouveau: switch over to new memory and vmm interfaces")
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Tested-by: Thierry Reding <treding@nvidia.com>