monitor-manager: Set switch config as LINEAR when ensuring configuration

It is linear config manager created when ensuring configuration. However, the switch config is not set as LINEAR, but left as UNKNOWN. This leads switch mode OSD always "Join Displays" icon, rather than the next icon which is "External Only" after connect an external display and press Super+P once at first time since mutter starts execution.

This patch sets the switch config as LINEAR when ensuring configuration. Then, the initial state of switch mode OSD will be "Join Displays". It can jump to next icon "External Only" correctly after Super+P.

This is a regression introduced by 149e4d69.

Fixes: #1362 (closed)