obj_priv->write_domain is "write domain if the GPU went idle now", not
"write domain at this moment."  By postponing the clear, we confused the
concept, required more storage, and potentially emitted more flushes than
are required.
Signed-off-by: NEric Anholt <eric@anholt.net>
Signed-off-by: NDave Airlie <airlied@redhat.com>