dri2: Call drm_queue_handle_deferred in dri2_deferred_event
drm_queue_handler just puts the event on the signalled list; without calling drm_queue_handle_deferred, actual processing of the event may be delayed indefinitely, e.g. until another event arrives from the kernel.
This could result in DRI2 clients hanging during DPMS off.
Fixes: 739181c8 "Add amdgpu_drm_handle_event wrapper for drmHandleEvent"