backport xf86/logind: fix missing call to vtenter if the platform device is not paused

Backport 2 commits to fix black screen issue with Nvidia, introduced in xserver-21.1.2

They fix #1271 (closed) and #1269 (closed)

If there is one platform device, which is not paused nor resumed, systemd_logind_vtenter() will never get called. This break suspend/resume, and switching to VT on system with Nvidia proprietary driver.

This is a regression introduced by f5bd0396

So now call systemd_logind_vtenter() if there are no paused platform devices.

Fix compilation error when built without logind/platform bus

This was introduced by commit 8eb1396d