drm/virtio: Fix driver removal with disabled KMS

drm/virtio: Fix driver removal with disabled KMS

DRM atomic and modesetting aren't initialized if virtio-gpu driver built
with disabled KMS, leading to access of uninitialized data on driver
removal/unbinding and crashing kernel. Fix it by skipping shutting down
atomic core with unavailable KMS.

Fixes: 72122c69d717 ("drm/virtio: Add option to disable KMS support")
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Tested-by: Ryosuke Yasuoka <ryasuoka@redhat.com>
Reviewed-by: Ryosuke Yasuoka <ryasuoka@redhat.com>
Link: https://patch.msgid.link/20260604122743.13383-1-dmitry.osipenko@collabora.com