Remove winsys->wait_idle(), subsumed by pipe->flush().

Remove winsys->wait_idle(), subsumed by pipe->flush().

Connect intel_i915_batch_finish() into i915_winsys, just like intel_i915_batch_flush().
Call i915_winsys->batch_finish() in response to pipe->flush(PIPE_FLUSH_WAIT).
Now all the batchbuffer/fence code is in one place and a little cleaner.