cleanup: Really stop using G_TYPE_INSTANCE_GET_PRIVATE()

Commit 4259cfd4 missed some occurences, move those to the generated
get_instance_private() functions as well.

GNOME/mutter!691
5 jobs for !691 with more-instance-private in 7 minutes and 40 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Review
passed #363362
check-commit-log

00:00:40

 
  Build
passed #363363
build-mutter

00:03:45

passed #363364
build-without-native-backend

00:03:34

 
  Test
passed #363366
can-build-gnome-shell

00:01:53

passed #363365
test-mutter

00:03:14