VM_BIND locking document

[Thomas Hellström]

Danilo, Boris

As you might know we have a task to complete a VM_BIND locking document 
including userptr gating the Xe driver upstreaming. I have version 3 of 
that document out as a patch

https://patchwork.freedesktop.org/series/122508/

updated with the latest gpuvm changes, including the fine-grained 
locking plus userptr locking as done in the Xe driver.

It would be greatly appreciated if you could read through and come with 
suggestions / objections / acks.  The idea is this document will simply 
be updated as drm_gpuvm evolves. In particular the userptr area 
currently follows Xe locking and probably needs to be adjusted if / when 
we get userptr in drm_gpuvm.

Thanks,

Thomas