drm/i915: Do not unlock upon error in i915_gem_idle()

We never took the lock ourselves and all callers expect the struct_mutex
to be locked upon return (be it success or error), thereore dropping the
lock along the error paths looks to be a vestigial error from

commit db1b76ca
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Tue Jul 9 16:51:37 2013 +0200

    drm/i915: don't frob mm.suspended when not using ums
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -4232,16 +4232,13 @@ i915_gem_idle(struct drm_device *dev)
 	drm_i915_private_t *dev_priv = dev->dev_private;
 	int ret;
 
-	if (dev_priv->ums.mm_suspended) {
-		mutex_unlock(&dev->struct_mutex);
+	if (dev_priv->ums.mm_suspended)
 		return 0;
-	}
 
 	ret = i915_gpu_idle(dev);
-	if (ret) {
-		mutex_unlock(&dev->struct_mutex);
+	if (ret)
 		return ret;
-	}
+
 	i915_gem_retire_requests(dev);
 
 	/* Under UMS, be paranoid and evict. */