drm/i915: Fix missing unlock on error in i915_ppgtt_info()

Add the missing unlock before return from function i915_ppgtt_info()
in the error handling case.

Fixes: 1d2ac40(drm: Protect dev->filelist with its own mutex)
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1465861320-26221-1-git-send-email-weiyj_lk@163.com
(cherry picked from commit b021248)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/i915_debugfs.c

@@ -2365,16 +2365,16 @@ static int i915_ppgtt_info(struct seq_file *m, void *data)
 		task = get_pid_task(file->pid, PIDTYPE_PID);
 		if (!task) {
 			ret = -ESRCH;
-			goto out_put;
+			goto out_unlock;
 		}
 		seq_printf(m, "\nproc: %s\n", task->comm);
 		put_task_struct(task);
 		idr_for_each(&file_priv->context_idr, per_file_ctx,
 			     (void *)(unsigned long)m);
 	}
+out_unlock:
 	mutex_unlock(&dev->filelist_mutex);
 
-out_put:
 	intel_runtime_pm_put(dev_priv);
 	mutex_unlock(&dev->struct_mutex);