opengl: gbm: Adds missing unrefs for gl context and drm node release in finalizer

This adds a few missing gst_object_unref calls for the opengl context in
gstglwindow_gbm_egl.c, as well as the missing close call for the
drm node fd in gst_gl_display_gbm_finalize.
3 jobs for glcontext-headless-gbm-cleanup in 4 minutes and 9 seconds (queued for 1 minute and 14 seconds)
latest
Status Name Job ID Coverage
  Manifest
passed manifest #48204

00:00:21

 
  Build
passed build android arm64 api28 #48206

00:02:23

passed build fedora x86_64 #48205

00:01:25