Fix FFmpeg component-specific logging to hide other log levels

Currently only log messages at AV_LOG_DEBUG level are hidden by disabling component-specific logging for FFmpeg.

This change makes it so that unless FFmpeg component-specific logging is turned on, log messages at AV_LOG_WARNING and AV_LOG_VERBOSE levels will also be hidden.

Fixes [#15537] (http://trac.kodi.tv/ticket/15537).