evdev: Initialize keymap before device manager

The initialization routines of ClutterDeviceManagerEvdev assume
that backend->keymap is already initialized. However, the current
code initializes backend->keymap after ClutterDeviceManagerEvdev,
making it crash on startup.

Fix that by initializing backend->keymap before the device manager.

#451
2 jobs for gbsneto/init-keymap in 4 minutes and 44 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #200363
build-mutter

00:00:59

 
  Test
passed #200364
test-mutter

00:03:44