fix DMA ops layering violations in vmwgfx

From: Christoph Hellwig
Date: Sat Jan 05 2019 - 03:02:07 EST

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.