st/nine: Fix crash when deleting non-implicit swapchain

st/nine: Fix crash when deleting non-implicit swapchain

The implicit swapchains are destroyed when the device instance is
destroyed. However for non-implicit swapchains, it is not the case,
and the application can have kept an reference on the swapchain
buffers to reuse them.

Fixes problems with battle.net launcher.

Cc: "10.4" <mesa-stable@lists.freedesktop.org>
Tested-by: Nick Sarnie <commendsarnex@gmail.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: David Heidelberg <david@ixit.cz>
Signed-off-by: Axel Davy <axel.davy@ens.fr>