UPSTREAM: drm: virtio-gpu: ensure plane is flushed to host on atomic update

This fixes drawing updates when updating planes with atomic API.

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>
(cherry picked from commit bd17d1c77c2ae3bb988a256ea5bda6d405b62b56)

Change-Id: Ib688992f35a775e2c90640387eceaf343df846de
Signed-off-by: Alistair Strachan <astrachan@google.com>