venus: fix shmem leak on vn_ring_destroy

Missed shmem unref when moving ring internals out of vn_instance.c.

Fixes: d1e29b7557 ("venus: move ring shmem into vn_ring")
Signed-off-by: Ryan Neph <ryanneph@google.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27125>

src/virtio/vulkan/vn_ring.c

@@ -346,6 +346,7 @@ vn_ring_destroy(struct vn_ring *ring)
       vk_free(alloc, submit);
 
    vn_cs_encoder_fini(&ring->upload);
+   vn_renderer_shmem_unref(ring->instance->renderer, ring->shmem);
 
    mtx_destroy(&ring->mutex);