dma-fence: Store the timestamp in the same union as the cb_list

The timestamp and the cb_list are mutually exclusive, the cb_list can
only be added to prior to being signaled (and once signaled we drain),
while the timestamp is only valid upon being signaled. Both the
timestamp and the cb_list are only valid while the fence is alive, and
as soon as no references are held can be replaced by the rcu_head.

By reusing the union for the timestamp, we squeeze the base dma_fence
struct to 64 bytes on x86-64.

v2: Sort the union chronologically

Suggested-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Christian König <christian.koenig@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>.
Link: https://patchwork.freedesktop.org/patch/msgid/20190817153022.5749-1-chris@chris-wilson.co.uk

drivers/gpu/drm/vmwgfx/vmwgfx_fence.c

@@ -184,6 +184,9 @@ static long vmw_fence_wait(struct dma_fence *f, bool intr, signed long timeout)
 
 	spin_lock(f->lock);
 
+	if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &f->flags))
+		goto out;
+
 	if (intr && signal_pending(current)) {
 		ret = -ERESTARTSYS;
 		goto out;