drm/i915: Move flushing list cleanup from flush request retire to request emit.

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: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>