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 #147541
xserver-1.13

00:00:24

passed #147542
xserver-1.14

00:00:38

passed #147543
xserver-1.15

00:00:27

passed #147544
xserver-1.16

00:00:44

passed #147545
xserver-1.17

00:00:25

passed #147546
xserver-1.18

00:00:24

passed #147547
xserver-1.18-clang

00:00:26

passed #147548
xserver-1.19

00:00:24

passed #147549
xserver-1.19-clang

00:00:43

passed #147539
xserver-1.20

00:00:40

passed #147540
xserver-1.20-clang

00:00:26