drm/bridge/synopsys: dsi: missing post disable

Sometime the post_disable function is missing (not registered).

Signed-off-by: Yannick Fertré <yannick.fertre@st.com>
Reviewed-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1579602296-7683-1-git-send-email-yannick.fertre@st.com