[Intel-gfx] Patch sequence to clear GTT and retry on -ENOMEM

[Keith Packard]

This patch sequence adds the ability to retry pinning in execbuffer when one
of the pin operations returns -ENOMEM. We unpin all of the buffers and then
clear the GTT before trying again, with the hope that we'll eliminate any
fragmentation this way.

Along the way, there are patches which finish the work of splitting the
various domain setting functions apart. This sequence renames set_domain to
set_to_gpu_domain reflecting its new limited role.

There is also a patch which eliminates any potential interruption between
the new domain computation and the flush operations needed to effect those
new domain changes.

With this sequence in place, I can (occasionally) run virtual forbidden
city, an application which routinely uses 100MB+ of aperture space for a
single operation.