[Intel-gfx] Derive requests from dma-buf fence

[Chris Wilson]

This is the minimal conversion to derive struct i915_gem_request from
dma-buf fences. They are still only used privately (i.e. the requests
are not attached to the dma-buf or any reservation object so not exposed
to any other driver or userspace) and so we defer the issue of RCU
access to the request until later - until we have annotations over own
access to the request so that we can catch RCU issues.

This series just looks at the issues surrounding using requests as
fences, plus a bonus patch aimed at BAT/bsw.
-Chris