drm/i915/tracepoints: Rename i915_gem_request_notify

i915_gem_ring_notify is more appropriate since we do not have
the request information at this point, but it is simply a
signal from the engine that some request has been completed.

v2:
  * Always trace and log if there were any waiters.
  * Rename to intel_engine_notify. (Chris Wilson)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>