No, you do not need install SSAS instance together with Power BI Report Server.
Power BI Report installation includes internal and hidden SSAS service in special disk-less mode, but you it's fully integrated, do not take care about it (if standard limitations of PBIRS import models are enough for you)
BUT, if you want little more:
-if you need models larger than 1 or 2GB
-if you want partitions and incremental processing
-if you make more reports based on single model
In these cases you need install SSAS instance (and create models in VisualStudio SSDT), but then, please, install SSAS on separated server (not on same server together with PBIRS), and use Power BI reports with live connection to SSAS.
