[Intel-gfx] A bunch of random execbuffer patches

[Chris Wilson]

So Daniel likes the "Tighten the checks for invalid relocation domains"
so much he wrote a i-g-t for it. Here is the patch to fix it - only I
had it inside a patch series to adjust the execbuffer API to reduce the
relocation overhead, which in turn requires a couple of other patches.
Then I threw in a couple of patches that have been baking nearby for
good measure...

It is the last patch to allow userspace to assume a known offset for a
particular bo that is both the most contentious and pulls in a bunch of
earlier patches - but those should be useful in their own right.

Please review, and look at the proposed changes to execbuffer carefully.
-Chris