Don't set up black scanout buffer if LeaveVT is called from CloseScreen

Avoids a crash described in
xorg/driver/xf86-video-amdgpu!43 (comment 223718)

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
12 jobs for AMDGPULeaveVT_KMS-CloseScreen-bail in 2 minutes and 41 seconds (queued for 21 seconds)
Status Job ID Name Coverage
  Docker Image
passed #637426
debian-testing

00:00:24

 
  Build
passed #637429
xserver-1.13

00:00:38

passed #637430
xserver-1.14

00:00:24

passed #637431
xserver-1.15

00:00:23

passed #637432
xserver-1.16

00:00:56

passed #637433
xserver-1.17

00:00:24

passed #637434
xserver-1.18

00:00:41

passed #637435
xserver-1.18-clang

00:00:26

passed #637436
xserver-1.19

00:00:39

passed #637437
xserver-1.19-clang

00:00:50

passed #637427
xserver-1.20

00:00:53

passed #637428
xserver-1.20-clang

00:00:50