[PATCH 0/4] Fix DMA ops layering violations in vmwgfx v2

This is a patch set from Christoph Hellwig. Patch 2 has been updated by me to not remove checks for TTM coherent pool presence. Ideally, we should query TTM for this instead and I'll have a patch set intended for 5.1 so that we can do that. But I don't want to introduce cross-module API additions outside of a merge window.

Original patch set description: vmwgfx has been doing some odd checks based on DMA ops which rely on deep DMA mapping layer internals, and I think the changes in Linux 4.21 finally broke most of these implicit assumptions.

The real fix is in patch 3, but I think the others are important to make it clear what is actually going on.