Cursor movement becomes synchronized with main content updates after VT switch

Description

After switching out to a different VT and back to the one that Mutter/gnome-shell controls, cursor movement is no longer independent of content updates and remains so until the session is restarted. Essentially, the main user-visible benefit introduced in !2777 (merged) is lost.

The following error is shown when switching out of the VT:

Failed to determine deadline: drmWaitVBlank failed: Operation not permitted

Root cause analysis

The error above results in priv->deadline_timer_inhibited being set to TRUE in main/src/backends/native/meta-kms-impl-device.c#L1622-1629.

After that, priv->deadline_timer_inhibited remains set to TRUE for the entire lifetime of the MetaKmsImplDevice. As long as it's set to TRUE, the deadline timer is not armed and cursor updates are no longer done on their own schedule independent of the main thread.

I'll upload a MR to fix this issue.