drm/i915/ringbuffer: Exclude last 2 cachlines of ring on 845g

The 845g shares the errata with i830 whereby executing a command
within 2 cachelines of the end of the ringbuffer may cause a GPU hang.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Cc: stable@kernel.org
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -1038,7 +1038,7 @@ int intel_init_ring_buffer(struct drm_device *dev,
 	 * of the buffer.
 	 */
 	ring->effective_size = ring->size;
-	if (IS_I830(ring->dev))
+	if (IS_I830(ring->dev) || IS_845G(ring->dev))
 		ring->effective_size -= 128;
 
 	return 0;