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"
Reviewed-by: Aaron Liu <aaron.liu@amd.com>
Tested-by: Aaron Liu <aaron.liu@amd.com>