On boot, i915.ko panics when "nomodeset" or "i915.modeset=-1" kernel option is set.
Workaround: add bogus "i915.crash=0" option to not load i915.ko .
Patch below: properly check the NULL pointer dereference.

diff -up linux-2.6.32-573.3.1.el6.emu686.v19.i586/drivers/gpu/drm/i915/intel_ringbuffer.c.v19+ linux-2.6.32-573.3.1.el6.emu686.v19.i586/drivers/gpu/drm/i915/intel_ringbuffer.c
--- linux-2.6.32-573.3.1.el6.emu686.v19.i586/drivers/gpu/drm/i915/intel_ringbuffer.c.v19+	2015-08-10 22:16:43.000000000 +0900
+++ linux-2.6.32-573.3.1.el6.emu686.v19.i586/drivers/gpu/drm/i915/intel_ringbuffer.c	2015-09-18 13:25:30.000000000 +0900
@@ -1821,12 +1821,18 @@ error:
 
 void intel_cleanup_ring_buffer(struct intel_engine_cs *ring)
 {
-	struct drm_i915_private *dev_priv = to_i915(ring->dev);
-	struct intel_ringbuffer *ringbuf = ring->buffer;
+	struct drm_i915_private *dev_priv;
+	struct intel_ringbuffer *ringbuf;
 
+	/* do not dereference NULL! */
+	if (ring == NULL)
+		return;
 	if (!intel_ring_initialized(ring))
 		return;
 
+	dev_priv = to_i915(ring->dev);	/*ring->dev->private_*/
+	ringbuf = ring->buffer;
+
 	intel_stop_ring_buffer(ring);
 	WARN_ON(!IS_GEN2(ring->dev) && (I915_READ_MODE(ring) & MODE_IDLE) == 0);