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_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
index c649e0f..49b3ebc 100644
--- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
@@ -203,9 +203,9 @@
 	cd->invalidate_domains |= invalidate_domains;
 	cd->flush_domains |= flush_domains;
 	if (flush_domains & I915_GEM_GPU_DOMAINS)
-		cd->flush_rings |= obj->ring->id;
+		cd->flush_rings |= intel_ring_flag(obj->ring);
 	if (invalidate_domains & I915_GEM_GPU_DOMAINS)
-		cd->flush_rings |= ring->id;
+		cd->flush_rings |= intel_ring_flag(ring);
 }
 
 struct eb_objects {