We must be able to reset the GPU while we are waiting on it to perform
an eviction (unbinding an active vma). So attach a spinning request to a
target vma and try and it evict it from a thread to see if that blocks
indefinitely.

v2: Add a wait for the thread to start just in case that takes more than
10ms...
v3: complete() not completion_done() to signal the completion.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180716134009.13143-1-chris@chris-wilson.co.uk