drm/bridge: Add a devm_ allocator for panel bridge.

This will let drivers reduce the error cleanup they need, in
particular the "is_panel_bridge" flag.

v2: Slight cleanup of remove function by Andrzej

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Philippe Cornu <philippe.cornu@st.com>
Tested-by: Philippe Cornu <philippe.cornu@st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170718210510.12229-2-eric@anholt.net