drm/i915: Remove the global cache shrink & rcu barrier on allocation failure

Earlier, we reasoned that having idled the gpu under mempressure, that
would be a good time to trim our request slabs in order to perform the
next request allocation. We have stopped performing the global operation
on the device (no idling) and wish to make the allocation failure
handling more local, so out with the global barrier that may take a long
time.

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/20181005080300.9908-2-chris@chris-wilson.co.uk