drm/i915: Rename dev_priv->ring to dev_priv->render_ring.

With the advent of the BSD ring, be clear about which ring this is.
The docs are pretty consistent with calling this the Render engine at
this point.
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 4f2f5f8..95dbe56 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -4378,7 +4378,7 @@
 
 	mutex_lock(&dev->struct_mutex);
 
-	if (dev_priv->mm.suspended || dev_priv->ring.ring_obj == NULL) {
+	if (dev_priv->mm.suspended || dev_priv->render_ring.ring_obj == NULL) {
 		mutex_unlock(&dev->struct_mutex);
 		return 0;
 	}