venus: add a wsi image log

This is helpful to tell which path is taken:
1. explicit modifier: legacy_scanout=0, prime_blit=0
2. prime blit: legacy_scanout=0, prime_blit=1
3. legacy scanout: legacy_scanout=1, prime_blit=0

To be noted, venus doesn't advertise legacy scanout support, but we
implicitly support it for gamescope compatibility.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/38457>

src/virtio/vulkan/vn_wsi.c

@@ -140,6 +140,10 @@ vn_wsi_create_image(struct vn_device *dev,
       return result;
 
    img->wsi.is_prime_blit_src = wsi_info->blit_src;
+   if (VN_DEBUG(WSI)) {
+      vn_log(dev->instance, "%s: legacy_scanout=%d, prime_blit=%d", __func__,
+             wsi_info->scanout, wsi_info->blit_src);
+   }
 
    *out_img = img;
    return VK_SUCCESS;