vkr: some further fixes after supporting external memory opaque_fd

If there's external memory import, then the virgl resource has to exist already. We don't need to force export since the resource_map will use the existing virgl resource directly.

If the client explicitly asks for dma_buf export, without the real dma_buf export support, we must fallback to use gbm bo import path, instead of using opaque fd export.

/cc @rakko