radv: fix destroying the syncobj when exporting a fence FD
It's invalid and the temporary syncobj was never actually destroyed. Cc: 20.1 <mesa-stable@lists.freedesktop.org> Signed-off-by:Samuel Pitoiset <samuel.pitoiset@gmail.com> Reviewed-by:
Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl> Part-of: <mesa/mesa!5921> (cherry picked from commit 8aa9d0ac)