wl/shm: don't fetch formats if not requested.

This aligns the code with the drm format code

Acked-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Simon Ser <contact@emersion.fr>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11945>

src/vulkan/wsi/wsi_common_wayland.c

@@ -418,6 +418,9 @@ static void
 shm_handle_format(void *data, struct wl_shm *shm, uint32_t format)
 {
    struct wsi_wl_display *display = data;
+   if (display->swrast.formats.element_size == 0)
+      return;
+
    wsi_wl_display_add_wl_shm_format(display, &display->swrast.formats, format);
 }