drm/i915: Don't let an oops get triggered from irq_emit without dma init.

Userland is broken if it's trying this, but we also shouldn't allow oopses.
Signed-off-by: Eric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/i915_irq.c

@@ -406,7 +406,7 @@ int i915_irq_emit(struct drm_device *dev, void *data,
 	drm_i915_irq_emit_t *emit = data;
 	int result;
 
-	if (!dev_priv) {
+	if (!dev_priv || !dev_priv->ring.virtual_start) {
 		DRM_ERROR("called with no initialization\n");
 		return -EINVAL;
 	}