drm/virtio: use damage info for display updates.

v2: remove shift by src_{x,y}, drm_atomic_helper_damage_merged()
    handles that for us (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/20191212125346.8334-4-kraxel@redhat.com