Convert the driver to dri_bufmgr interface and enable TTM.
This is currently believed to work but be a significant performance loss. Performance recovery should be soon to follow. The dri_bo_fake_disable_backing_store() call was added to allow backing store disable like bufmgr_fake.c did, which is a significant performance win (though it's missing the no-fence-subdata part). This commit is a squash merge of the 965-ttm branch, which had some history I wanted to avoid pulling due to noisiness and brokenness at many points for git-bisecting.
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