drm/stm: ltdc: remove bridge from driver internal structure

With a call to drm_of_panel_bridge_remove() we could remove the bridge
without store it in ldtc internal driver structure.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Reviewed-by: Philippe Cornu <philippe.cornu@st.com>
Tested-by: Philippe Cornu <philippe.cornu@st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1506936888-23844-4-git-send-email-benjamin.gaignard@linaro.org