drm/virtio: skip set_scanout if framebuffer didn't change

v2: also check src rect (Chia-I Wu).

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20191212125346.8334-2-kraxel@redhat.com