ttm: merge ttm_backend & ttm_tt, introduce ttm dma allocator

[j.glisse@gmail.com]

So i did an overhaul of ttm_memory, i believe the simplification i did
make sense. See patch 5 for a longer explanation.

Thomas with the ttm_memory change the allocation of pages won't happen
if the accounting report that we are going over the limit and bo shrinker
failed to free any memory to make room.

The handling of dma32 zone is done as post pass of ttm memory accounting.

Regarding the pagefault comment i removed, it doesn't make sense anymore
because now we populate the whole page table in one shot. So there is
no more prefaulting few pages but a full prefaulting. Thought i can
add a comment stating that if you like.

For the ttm_tt_dma struct to hold page allocator specific informations
i think it can be done as an followup patch but if you prefer to have
that in this patchset let me know i will respin with such changes.

I am in the process of retesting this whole serie and especialy the
while memory accounting.

Cheers,
Jerome