Report Server SQL licenses

I'm about to install Power BI Report Server on a local machine. We have a Power BI P1 node so I understand I can install it on a 8 core machine.

I also understand I need to have a SQL Server somewhere to host the report server db. My question is about the licensing of this SQL server.

I attended a lot of webinars hosted by Microsoft since the launch of Report Server and in a previous one (when they were going GA), they mentionned that you could install a SQL server free of charge (include in the Report Server license) if it was only for the report server db. Now I can't find anything about this one the web or in the doc.

Can somebody confirm?