drm/bridge/synopsys: dsi: use adjusted_mode in mode_set

The "adjusted_mode" clock value (ie the real pixel clock) is more
accurate than "mode" clock value (ie the panel/bridge requested
clock value). It offers a better preciseness for timing
computations and allows to reduce the extra dsi bandwidth in
burst mode (from ~20% to ~10-12%, hw platform dependent).

Signed-off-by: Philippe Cornu <philippe.cornu@st.com>
Tested-by: Brian Norris <briannorris@chromium.org>
Reviewed-by: Yannick Fertré <yannick.fertre@st.com>
Tested-by: Yannick Fertré <yannick.fertre@st.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180125155504.8611-1-philippe.cornu@st.com