ttm: merge ttm_backend & ttm_tt, introduce ttm dma allocator [FULL]

[j.glisse at gmail.com]

Ok so here is full patchset, including nouveau support, Ben if you
could review (if change to nouveau in patch 7 are correct then others
change to nouveau are more than likely 100% correct :)).

So been tested on R7XX,EVERGREEN,CAICOS,CAYMAN with SWIOTLB. Also
tested on NV50. I still need to test AGP configuration but i am quite
confident that there is no regression for PCIE/PCI.

Cheers,
Jerome