drm/ttm: new TT backend allocation pool

[Christian König]

This replaces the spaghetti code in the two existing page pools.
    
First of all depending on the allocation size it is between 3 (1GiB) and
5 (1MiB) times faster than the old implementation.
    
It makes better use of buddy pages to allow for larger physical contiguous
allocations which should result in better TLB utilization at least for amdgpu.
    
Instead of a completely braindead approach of filling the pool with one CPU
while another one is trying to shrink it we only give back freed pages.
    
This also results in much less locking contention and a trylock free MM
shrinker callback, so we can guarantee that pages are given back to the system
when needed.
    
Downside of this is that it takes longer for many small allocations until the
pool is filled up. We could address this, but I couldn't find an use case
where this actually matters. And we don't bother freeing large chunks of pages
any more.
    
The sysfs files are replaced with a single module parameter, allowing users to
override how many pages should be globally pooled in TTM. This unfortunately
breaks the UAPI slightly, but as far as we know nobody ever depended on this.
   
Zeroing memory coming from the pool was handled inconsistently. The
alloc_pages() based pool was zeroing it, the dma_alloc_attr() based one wasn't.
The new implementation isn't zeroing pages from the pool either and only sets
the __GFP_ZERO flag when necessary.
    
The implementation has only 753 lines of code compared to the over 2600 of the
old one, and also allows for saving quite a bunch of code in the drivers since
we don't need specialized handling there any more based on kernel config.
  
Additional to all of that there was a neat bug with IOMMU, coherent DMA
mappings and huge pages which is now fixed in the new code as well.

Please review and comment,
Christian.