By default, you should only get landscape home screens if you:
- Connect your phone to an external video source via MHL (HDMI).
- Activate Media Link HD (requires HTC Media Link hardware).
I have encountered a "bug" once, where I was able to rotate my screen and get my home screen to rotate with it. Not sure what I did to cause it to happen, and it lasted until I needed to reboot the phone. Have not been able to get it to do so again since.
Alternatively, you can install one of the custom launchers from the play store. Some (or all?) of them will give you the option to enable home screen rotation. Just keep in mind that you won't be able to use any of HTC's widgets while using an alternate launcher.