backend-drm: make GBM optional

Make GBM optional in case GL renderer is disabled. This allows to
build Weston with DRM backend without Mesa dependencies.

Signed-off-by: Stefan Agner <stefan@agner.ch>
2 jobs for support-drm-without-gbm in 1 minute and 27 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Container Prep
passed #410331
container_prep

00:00:28

 
  Build
passed #410332
build-native-meson

00:00:58