drm/i915: Avoid oops when capturing NULL ring for inactive pinned buffers

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
diff --git a/drivers/gpu/drm/i915/i915_debugfs.c b/drivers/gpu/drm/i915/i915_debugfs.c
index 421b841..4fe49e0 100644
--- a/drivers/gpu/drm/i915/i915_debugfs.c
+++ b/drivers/gpu/drm/i915/i915_debugfs.c
@@ -572,9 +572,9 @@
 static const char *ring_str(int ring)
 {
 	switch (ring) {
-	case RING_RENDER: return "render";
-	case RING_BSD: return "bsd";
-	case RING_BLT: return "blt";
+	case RING_RENDER: return " render";
+	case RING_BSD: return " bsd";
+	case RING_BLT: return " blt";
 	default: return "";
 	}
 }