drm/i915: Use cmpxchg64 for 32b compatilibity

By using the double wide cmpxchg64 on 32bit, we can use the same
algorithm on both 32/64b systems.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201211110310.22740-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_active.c

@@ -159,8 +159,7 @@ __active_retire(struct i915_active *ref)
 		GEM_BUG_ON(ref->tree.rb_node != &ref->cache->node);
 
 		/* Make the cached node available for reuse with any timeline */
-		if (IS_ENABLED(CONFIG_64BIT))
-			ref->cache->timeline = 0; /* needs cmpxchg(u64) */
+		ref->cache->timeline = 0; /* needs cmpxchg(u64) */
 	}
 
 	spin_unlock_irqrestore(&ref->tree_lock, flags);
@@ -256,7 +255,6 @@ static struct active_node *__active_lookup(struct i915_active *ref, u64 idx)
 		if (cached == idx)
 			return it;
 
-#ifdef CONFIG_64BIT /* for cmpxchg(u64) */
 		/*
 		 * An unclaimed cache [.timeline=0] can only be claimed once.
 		 *
@@ -267,9 +265,8 @@ static struct active_node *__active_lookup(struct i915_active *ref, u64 idx)
 		 * only the winner of that race will cmpxchg return the old
 		 * value of 0).
 		 */
-		if (!cached && !cmpxchg(&it->timeline, 0, idx))
+		if (!cached && !cmpxchg64(&it->timeline, 0, idx))
 			return it;
-#endif
 	}
 
 	BUILD_BUG_ON(offsetof(typeof(*it), node));