drm/bridge: make drm_panel_bridge_remove more robust

Make sure that bridge parameter is not NULL and can be safely
cast into a panel_bridge 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/1506936755-23625-2-git-send-email-benjamin.gaignard@linaro.org