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)