main: Avoid destroying wet_output before output destroy callback is called

In using drm-backend, if drm_output destroying is pending, wet_output
destroy before that destroy callback function is called. So freed
wet_output is refered in this function. This patch fix this issue.
2 jobs for fix-output-destroy in 49 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Container Prep
passed #287464
container_prep

00:00:18

 
  Build
passed #287465
build-native-meson

00:00:30