Meson: Set some options back to auto by default

Some options got enabled by default since that commit: 40371ff9

While the commit message example makes perfect sense to enable bad by default, I don't think the same reasoning applies to libav, devtools, ges and rtsp_server.

I think it is important to keep a build with -Dauto_features=disabled minimal, especially not pulling external dependencies. For example, with libav being enabled by default, Meson builds FFmpeg subproject even when disabling auto features.

It is fine to keep bad enabled by default because itself has auto features for every plugins, when auto features are disabled it only build libraries that does not have external deps.