You do not need to configure a separate SSAS instance to support Power BI Report server. The only pre-requisite is to have access to a SQL Server relational engine instance so that the Report Server can create a database in which to store it's metadata (usually called ReportServer by default) and a temp database (usually called ReportServerTempDB).
Power BI Report Server does come with it's own internal SSAS engine that it uses for running the data models inside pbix reports, but you do not need to install or manage that instance it is effectively embedded inside the Report Server.
You can also connect reports to existing tabular or multi-dim SSAS instances if you have them, but this is optional. If you do not have an existing SSAS instance you do not need to install one to use Power BI Report Server.
