igt/gem_exec_nop: Restore check on available signal rings

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
diff --git a/tests/gem_exec_nop.c b/tests/gem_exec_nop.c
index 570f57b..440e357 100644
--- a/tests/gem_exec_nop.c
+++ b/tests/gem_exec_nop.c
@@ -539,6 +539,7 @@
 			engines[nengine++] = n;
 		}
 	} else {
+		gem_require_ring(fd, ring_id);
 		engines[nengine++] = ring_id;
 	}
 	igt_require(nengine);