drm/i915: release struct_mutex on the i915_gem_init_hw fail path

Release struct_mutex if init_rings() fails.

This is a regression introduced in
commit 35a57ffbb10840af219eeaf64718434242bb7c76
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Nov 20 00:33:07 2014 +0100

    drm/i915: Only init engines once

Reported-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -4860,22 +4860,18 @@ int i915_gem_init(struct drm_device *dev)
 	}
 
 	ret = i915_gem_init_userptr(dev);
-	if (ret) {
-		mutex_unlock(&dev->struct_mutex);
-		return ret;
-	}
+	if (ret)
+		goto out_unlock;
 
 	i915_gem_init_global_gtt(dev);
 
 	ret = i915_gem_context_init(dev);
-	if (ret) {
-		mutex_unlock(&dev->struct_mutex);
-		return ret;
-	}
+	if (ret)
+		goto out_unlock;
 
 	ret = dev_priv->gt.init_rings(dev);
 	if (ret)
-		return ret;
+		goto out_unlock;
 
 	ret = i915_gem_init_hw(dev);
 	if (ret == -EIO) {
@@ -4887,6 +4883,8 @@ int i915_gem_init(struct drm_device *dev)
 		atomic_set_mask(I915_WEDGED, &dev_priv->gpu_error.reset_counter);
 		ret = 0;
 	}
+
+out_unlock:
 	mutex_unlock(&dev->struct_mutex);
 
 	return ret;