lockscreen: Always cancel session

When on the keypad and moving the surface slightly and letting it snap back into place it happens that we see the carousel_page_change_cb () with the same value as before.

In the case of POS_UNLOCK this results in phog_greetd_session_create() being called on an already created session. This results in show_info_page() being invoked.

The result from an end users perspective is that when typing the passcode quickly phog often switches back to the overview page.

Fix this by always cancelling the session when we enter the function.

I hope I got this right. At least I can unlock the session with phog as quickly as on phosh's lockscreen with this applied.