drm/ttm: Pool allocator simplification and sysfs interface

[Pauli Nieminen <suokkos@gmail.com>]

This patch series simplifies pool management code and adds sysfd interface
to control pool functionality from sysfs.

Pool size reduction only happens when mm shrinker request for free
pages. To prevent pool from wasting too much free memory there is
user configureable maximum size. If pool tries to grow to larger
sizes than maximum pages are directly freed instead of putting to pool.

Number of pages allocated in refill is user configureable. Default
is same as number of struct page pointers fits to a page. 
Allocation size is limited so that user can't set insanely large numbers.
warning is generated when large allocation size doesn't make sense for
performance.

If ttm is running close to memory limit pool can't add more than
allocation_size pages to the memory use. This is quarenteed with
rule that only one pool refill can be running at a time.

 

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