drm/bridge/synopsys: stop clobbering drvdata

Bridge drivers/helpers shouldn't be clobbering the drvdata, since a
parent driver might need to own this. Instead, let's return our
'dw_mipi_dsi' object and have callers pass that back to us for removal.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Reviewed-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Archit Taneja <architt@codeaurora.org>
Acked-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171128010538.119114-1-briannorris@chromium.org