v4l2: allocator: Don't close foreign dmabuf

Imported dmabuf are not being duped, so they should never be closed. Instead, we ensure their live time by having strong reference on their original buffer. This should fix potential flickering due to dmabuf being closed too early.

I've stumbled across this code while reviewing code. Some testing needed, but it is obviously not correct. I remember dropping all the dmabuf dups, as these were not needed, this is probably left over.