drm/nouveau/nouveau/ttm: fix tiled system memory with Maxwell

Add Maxwell to the switch statement that sets node->memtype, otherwise
all tiling information is ignored for buffers in system memory.

While we are at it, make that switch statement explicitly complain the
next time we meet a non-handled card family.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>