drm/i915: Remove intel_ring.last_retired_head

Storing the position of the breadcrumb of the last retired request as
a separate last_retired_head is superfluous as we always copy that into
head prior to recalculation of the intel_ring.space.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170321102552.24357-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_debugfs.c

@@ -1938,9 +1938,8 @@ static int i915_gem_framebuffer_info(struct seq_file *m, void *data)
 
 static void describe_ctx_ring(struct seq_file *m, struct intel_ring *ring)
 {
-	seq_printf(m, " (ringbuffer, space: %d, head: %u, tail: %u, last head: %d)",
-		   ring->space, ring->head, ring->tail,
-		   ring->last_retired_head);
+	seq_printf(m, " (ringbuffer, space: %d, head: %u, tail: %u)",
+		   ring->space, ring->head, ring->tail);
 }
 
 static int i915_context_status(struct seq_file *m, void *unused)