clutter/evdev: Set the backend keymap before using it

The device manager evdev assumes the keymap is set, so better set it
before its creation.

Closes: GNOME/mutter#451
Fixes: 6de81b05 "evdev: Implement ClutterKeymap"
2 jobs for issue451 in 5 minutes and 40 seconds
Status Job ID Name Coverage
  Build
passed #200357
build-mutter

00:02:20

 
  Test
passed #200358
test-mutter

00:03:19