drm/ttm: stop pooling cached NUMA pages v2

drm/ttm: stop pooling cached NUMA pages v2

We only pool write combined and uncached allocations because they
require extra overhead on allocation and release.

If we also pool cached NUMA it not only means some extra unnecessary
overhead, but also that under memory pressure it can happen that
pages from the wrong NUMA node enters the pool and are re-used
over and over again.

This can lead to performance reduction after running into memory
pressure.

v2: restructure and cleanup the code a bit from the internal hack to
    test this.

Signed-off-by: Christian König <[email protected]>
Fixes: 4482d3c94d7f ("drm/ttm: add NUMA node id to the pool")
CC: [email protected]
Reviewed-by: Felix Kuehling <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]