monitor-manager: Don't use switch-config when ensuring configuration

Switch-configs are only to be used in certain circumstances (see
meta_monitor_manager_can_switch_config()) so when ensuring
configuration and attempting to create a linear configuration, use the
linear configuration constructor function directly without going via the
switch config method, otherwise we might incorrectly fall back to the
fallback configuration (only enable primary monitor).

This is a regression introduced by 6267732b.

Fixes: #342
2 jobs from master in 5 minutes and 43 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #167457
build-mutter

00:01:57

 
  Test
passed #167458
test-mutter

00:03:45