drm/stm: attach gem fence to atomic state

To properly synchronize with other devices the fence from the GEM
object backing the framebuffer needs to be attached to the atomic
state, so the commit work can wait on fence signaling.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Acked-by: Philippe Cornu <philippe.cornu@st.com>
Tested-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190712084228.8338-1-l.stach@pengutronix.de