drm/bochs: deinit bugfix

Check whenever mode_config was actually properly
initialized before trying to clean it up.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20200207124348.21641-1-kraxel@redhat.com