drm/virtio: add plane check

Use drm_atomic_helper_check_plane_state()
to sanity check the plane state.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Chia-I Wu <olvaffe@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190822094657.27483-1-kraxel@redhat.com