drm/virtio: drop framebuffer dirty tracking code

No need to have our own implementation,
atomic helpers can do it for us.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20190611125408.29421-1-kraxel@redhat.com