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>
Status Job ID Name Coverage
  Container Prep
passed #435900
container_prep

00:00:30

 
  Build
passed #435901
build-native-meson

00:00:53