fix DMA ops layering violations in vmwgfx

[Christoph Hellwig]

Hi Thomas,

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.