drm/panel: otm8009a: Fix backlight updates

Backlight updates was not working anymore since the good implementation
of the DSI low-power mode in the DSI host driver. After a longer
analysis, the backlight updates in DSI video mode require the DSI high-
speed mode.

Note: it is important to keep the DSI low-power mode for the rest of the
driver as init sequence, sleep in/out... DSI commands work in low-power
mode.

Signed-off-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180423141054.13128-2-philippe.cornu@st.com