radv: Update code pointer correctly if a variant is already created

radv: Update code pointer correctly if a variant is already created

This was the actual cause of GPU hangs fixed by 0fdd531457ec ("radv:
Fix pipeline cache locking issues"), since multiple threads would end
up trying to create the variants for a single entry.

Now that we're locking around the whole of this function, this isn't
really necessary (we either create all or none of the variants), but
fix this anyway in case things change later.

Signed-off-by: Alex Smith <asmith@feralinteractive.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
CC: 17.3 <mesa-stable@lists.freedesktop.org>