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: ba83a866 "Add radeon_drm_handle_event wrapper for
                     drmHandleEvent"
(Ported from amdgpu commit 09be74a3d1dd9604336d9a27f98d132b262dcbaf)
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>