xinput-calibrator: Fix service file to require display-manager.service

The xinput-calibrator.service had a requires to xserver-nodm.service
but in fact it ought to wait by any display-manager to be initialized.
We fix it changing the require to display-manager.service.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>