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)