drm/i915 disallow physical batchbuffers for KMS

Even the horrible gen3 XvMC code has learned to do this
right by the time xf86-video-intel releases learned to do
kernel modesetting. So we can just disallow this.
Acked-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -3567,7 +3567,8 @@ int i915_gem_init(struct drm_device *dev)
 		return ret;
 	}
 
-	/* Allow hardware batchbuffers unless told otherwise. */
+	/* Allow hardware batchbuffers unless told otherwise, but not for KMS. */
+	if (!drm_core_check_feature(dev, DRIVER_MODESET))
 		dev_priv->dri1.allow_batchbuffer = 1;
 	return 0;
 }