video: Merge video_stream_start/stop driver APIs

The video_stream_start/stop() APIs are counter-symetric and have
the same function signature. Also, the implementation logic for
those driver APIs is generally the same. Merge them to save memory
and code lines.

For the sake of simplicity, still keep the user APIs to preserve
backward compatibility with downstream applications.

Signed-off-by: Phi Bang Nguyen <phibang.nguyen@nxp.com>