drm/i915: switch ring->id to be a real id

[Dan Carpenter]

Hello Daniel Vetter,

The patch 96154f2faba5: "drm/i915: switch ring->id to be a real id" 
from Dec 14, 2011, leads to the following warning:
drivers/gpu/drm/i915/i915_debugfs.c:688 print_error_buffers()
	 error: err->ring is never equal to -1 (wrong type 0 - 15).

@@ -724,6 +724,7 @@ static void print_error_buffers(struct seq_file *m,
                           tiling_flag(err->tiling),
                           dirty_flag(err->dirty),
                           purgeable_flag(err->purgeable),
+                          err->ring != -1 ? " " : "",
                           ^^^^^^^^^^^^^^^
->ring is an unsigned bitfield so it's never equal to -1.

                           ring_str(err->ring),
                           cache_level_str(err->cache_level));

regards,
dan carpenter