drm/i915: fix up ring cleanup for the i830/i845 CS tlb w/a

It's not a good idea to also run the pipe_control cleanup.

This regression has been introduced whith the original cs tlb w/a in

commit b45305fc
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Mon Dec 17 16:21:27 2012 +0100

    drm/i915: Implement workaround for broken CS tlb on i830/845

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=64610
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@vger.kernel.org
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -534,9 +534,6 @@ cleanup_pipe_control(struct intel_ring_buffer *ring)
 	struct pipe_control *pc = ring->private;
 	struct drm_i915_gem_object *obj;
 
-	if (!ring->private)
-		return;
-
 	obj = pc->obj;
 
 	kunmap(sg_page(obj->pages->sgl));
@@ -544,7 +541,6 @@ cleanup_pipe_control(struct intel_ring_buffer *ring)
 	drm_gem_object_unreference(&obj->base);
 
 	kfree(pc);
-	ring->private = NULL;
 }
 
 static int init_render_ring(struct intel_ring_buffer *ring)
@@ -617,7 +613,10 @@ static void render_ring_cleanup(struct intel_ring_buffer *ring)
 	if (HAS_BROKEN_CS_TLB(dev))
 		drm_gem_object_unreference(to_gem_object(ring->private));
 
-	cleanup_pipe_control(ring);
+	if (INTEL_INFO(dev)->gen >= 5)
+		cleanup_pipe_control(ring);
+
+	ring->private = NULL;
 }
 
 static void