Added simulated external display in Developer Settings and can't boot now
You can try wiping cache and dalvik before entering the safe mode. A similar problem is addressed on an XDA thread here. 

If the above doesn't work, you can restore the previous backup (hopefully a NAndroid one). If that doesn't work, you can try flashing CM again. Finally, if none of the above works, you can do a full data wipe and factory reset and flash + restore again.

Yet another approach is manually editing the developer options database via ADB as discussed here. 

adb shell
sqlite3 /data/data/com.android.providers.settings/databases/settings.db
delete from global where name="overlay_display_devices";
reboot


If the above doesn't work, then as said in above XDA thread, delete the developer settings manually as follows (all system specific settings will be reset), first mount /data in recovery:

adb shell
cd /data/data/com.android.providers.settings/databases/
delete settings.db
reboot 


If nothing works, the problem could be something different than the secondary display.