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

Avoids a crash described in
https://gitlab.freedesktop.org/xorg/driver/xf86-video-amdgpu/merge_requests/43#note_223718



(Ported from amdgpu commit 5b8bc9fc505c551dcd9b0ed5ab835a49fa4f9fda)
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>