drm/panel: mantix: Tweak init sequence

We've seen some (non permanent) burn in and bad white balance
on some of the panels. Adding this bit from a vendor supplied
sequence fixes it.

Fixes: 72967d56 ("drm/panel: Add panel driver for the Mantix MLAF057WE51-X DSI panel")
Signed-off-by: Guido Günther <agx@sigxcpu.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/8451831b60d5ecb73a156613d98218a31bd55680.1605688147.git.agx@sigxcpu.org