layouts: don't double activate persp-mode
layouts: don't double activate persp-mode

If persp-mode is already activated, don't re-activate it. Should fix bug
where running dotspacemacs/sync-configuration-layers (SPC-f-e-R) resets
all the layouts.

tsonntag/dotemacs,tsonntag/dotemacs,tsonntag/dotemacs,tsonntag/dotemacs