radv/winsys: Fix memory leak.

radv/winsys: Fix memory leak.

Fix warning reported by Coverity Scan.

Resource leak (RESOURCE_LEAK)
leaked_storage: Variable handles going out of scope leaks the storage it
points to.

Fixes 485ea7d711fc ("radv/winsys: pass the buffer list via the CS ioctl for less CPU overhead")

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by:  Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6510>