`magit-display-process': fix displaying the process buffer

Commit e26c9c79 erroneously wrapped the call to `display-buffer'
inside the `unless', meaning if we *had* a buffer to display, the
code to display it *wouldn't* run, essentially causing the process
buffer to never be displayed when the BUFFER arg was non-nil, and
thus breaking `magit-process-popup-time'-based process buffer popup.

Fixes #887.

Signed-off-by: Pieter Praet <pieter@praet.org>