vkr: respect client request for dma_buf export

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.

Signed-off-by: Yiwei Zhang <zzyiwei@chromium.org>
Reviewed-by: Chad Versace <chadversary@chromium.org>
Part-of: <!825>
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