UPSTREAM: drm: virtio-gpu: get the fb from the plane state for atomic updates

When using the atomic API, plane->fb is not set when calling
virtio_gpu_plane_atomic_update. Use plane->state->fb instead.

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>
(cherry picked from commit 11c94ace5e25a1b6d12184b1a6f54ef4dc75aa62)

Change-Id: Ic5f4e3852fcd44af965d6a1cf784c38a34aee19e
Signed-off-by: Alistair Strachan <astrachan@google.com>