drm/bridge/synopsys: dsi: Add a warning msg on dsi read requests

The dcs/generic dsi read feature is not yet implemented so it
is important to warn the host_transfer() caller in case of
read operation requests.

Reviewed-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180125103800.1999-2-philippe.cornu@st.com