ttm: merge ttm_backend & ttm_tt, introduce ttm dma allocator

Hi,

So updated patchset, only patch 5 seen change since last set.
Last 3 patch are from your patchset, modified on top of mine.

Konrad so i added you dma pool allocator on top of that
and added support for it to radeon. All in all it's slightly
smaller than your patchset.

Biggest change is use of a list_head in ttm_tt to keep the
dma_page list inside the ttm_tt object allowing faster and
lot simpler deallocation of page.

I only briefly test this code, it seems ok so far. Did you
tested booting kernel with swiotlb=force and with your patchset ?
Because here it doesn't work. I still don't understand why
swiotlb want to create a bounce page when the page supplied
fit the constraint. Need to dig into kernel history to see if
there is any good reasons for that.

Otherwise i believe this whole patchset make things cleaner
and simpler for ttm.

Cheers,
Jerome Glisse