The input of the vacompositor may be DMA buffers. And in this case, the input
caps has the format=DMA_DRM, which can not be recognized by base video
aggregator class' find_best_format() function. So we need to override the
update_caps() virtual function.
Also we consider the DMA kind caps in negotiated_src_caps() for output.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5160>