vkr: some further fixes after supporting external memory opaque_fd

Yiwei Zhang requested to merge zzyiwei/virglrenderer:force-export-fix into master

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

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