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

... and add a helpr function for the places where we want a flag.

This way we can use ring->id to index into arrays.

v2: Resurrect the missing beautification-space Chris Wilson noted.
I'm moving this space around because I'll reuse ring_str in the next
patch.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
index 275ab6f..ab53edb 100644
--- a/drivers/gpu/drm/i915/i915_irq.c
+++ b/drivers/gpu/drm/i915/i915_irq.c
@@ -804,7 +804,7 @@
 		err->tiling = obj->tiling_mode;
 		err->dirty = obj->dirty;
 		err->purgeable = obj->madv != I915_MADV_WILLNEED;
-		err->ring = obj->ring ? obj->ring->id : 0;
+		err->ring = obj->ring ? obj->ring->id : -1;
 		err->cache_level = obj->cache_level;
 
 		if (++i == count)