drm/i915: Only defer freeing of fence callback when also using the timer

Without an accompanying timer (for internal fences), we can free the
fence callback immediately as we do not need to employ the RCU barrier
to serialise with the timer. By avoiding the RCU delay, we can avoid the
extra mempressure under heavy inter-engine request utilisation.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180115090643.26696-1-chris@chris-wilson.co.uk