Draft: v4l2: videodec: DMA_DRM caps support PoC

This is a proof-of-concept and not yet ready for production

This allows the Raspberry Pi 4 H264 decoder to negotiate caps with the new DMA_DRM API, notably enabling some GTK4 clients to use Wayland offloading to hardware planes.

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See also:

• https://blog.gtk.org/2023/11/15/introducing-graphics-offload/
• !5890 (merged)
• https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/6618