[Intel-gfx] Gratiutuous struct_mutex elimination

[Chris Wilson]

I threw in some patches to remove all but a few struct_mutexes, notable
remnants are execbuf (since we have the great reservation debate), perf
(it needs to figure out its locking) and gvt (I'm not sure what it is
using struct_mutex for, so left it).

The real meat of the patches is breaking the stranglehold of
struct_mutex from around requests, which I hope CI should be happy with,
except CI has been on vacation...
-Chris