[Nouveau] NV50: the tiled buffer object eviction problem

[Maarten Maathuis]

In short, we move out the low level content of a buffer object. In the
case of textures and such this is utterly useless. Still it is
accessed, because ttm sees no problem in using PL_SYSTEM or PL_TT
memory. What is the best way to let ttm know we don't really
appreciate that? A custom memory to evict to that cannot be mapped
maybe. Or an extension of PL_SYSTEM or PL_TT. Share your ideas.

Maarten.