plugins: use VA allocator by default on raw caps

Instead of using dmabuf allocator in source pad, when raw video caps
are negotiated, it uses VA allocator as before, since it is stable
in more use cases, for example transcoding, and more backend drivers.

Dmabuf allocator is only used when dmabuf caps feature is negotiated.

Part-of: <!352>
parent 13d090b5
Pipeline #173497 waiting for manual action with stages
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