drm/virtio: use drmm_mode_config_init

Use managed init call to simplify cleanup.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20200908070723.6394-2-kraxel@redhat.com