drm/bridge/synopsys: dsi: Add read feature

This patch adds the DCS/GENERIC DSI read feature.

Signed-off-by: Philippe Cornu <philippe.cornu@st.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180204213104.17834-1-philippe.cornu@st.com