Avoid loading FPP from database in fieldable_panels_panes_preprocess_panels_pane()

In fieldable_panels_panes_preprocess_panels_pane() it's loading the fieldable_panels_pane entity in question from the database again, even though it's already be present on the $vars.

In the normal case, this is wasteful, since you already have perfectly fine object already loaded up.

But changing this will also allow Panopoly to simplify it's preview generating code for FPPs. In the past, Panopoly would actually save FPPs to the database just so that they get rendered correctly. But then we're saving something before the user has even pressed save!

Instead, we'd like to just render an unsaved FPP entity, and this function is causing trouble by loading the FPP from the database.

I'll post a patch in a moment.