backend-drm: get handle in gbm specific code

Get GBM BO handle in GBM specific code. This allows to compile
drm_output_set_cursor without GBM.

Signed-off-by: Stefan Agner <stefan@agner.ch>
2 jobs for support-drm-without-gbm-preparation in 1 minute and 33 seconds (queued for 9 seconds)
Status Job ID Name Coverage
  Container Prep
passed #407553
container_prep

00:00:25

 
  Build
passed #407554
build-native-meson

00:01:08