wsi/x11: cleanup properly after mit shm paths are used.

This path was being skipped on sw + has_mit_shm, when it
shouldn't be.

Fixes: a069b4e9b9 ("vulkan/wsi/sw: add mit-shm support for pixmap allocation")
Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13217>

src/vulkan/wsi/wsi_common_x11.c

@@ -1512,7 +1512,7 @@ x11_image_finish(struct x11_swapchain *chain,
 {
    xcb_void_cookie_t cookie;
 
-   if (!chain->base.wsi->sw) {
+   if (!chain->base.wsi->sw || chain->has_mit_shm) {
       cookie = xcb_sync_destroy_fence(chain->conn, image->sync_fence);
       xcb_discard_reply(chain->conn, cookie.sequence);
       xshmfence_unmap_shm(image->shm_fence);