backends/monitor-manager: Improve tiled monitor handling (LP: 2131575)

When the main tile in the topology is missing, and a user connects a tiled monitor, they will have a black / unusable screen on that monitor. This can occur in cases where a monitor has multiple physical connections, if a user connects to a port on the monitor that is not the main tile.

Signed-off-by: Mitchell Augustin mitchell.augustin@canonical.com