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>
11 jobs for dri2-handle_deferred in 1 minute and 52 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed xserver-1.13 #147541

00:00:24

passed xserver-1.14 #147542

00:00:38

passed xserver-1.15 #147543

00:00:27

passed xserver-1.16 #147544

00:00:44

passed xserver-1.17 #147545

00:00:25

passed xserver-1.18 #147546

00:00:24

passed xserver-1.18-clang #147547

00:00:26

passed xserver-1.19 #147548

00:00:24

passed xserver-1.19-clang #147549

00:00:43

passed xserver-1.20 #147539

00:00:40

passed xserver-1.20-clang #147540

00:00:26