drm/panel: mantix: Allow to specify default mode for different panels

This can be used to use different modes for differnt panels via OF
device match.

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/2580dba34c95a8159c1bdfd07604fbb8dbd0ad8c.1605688147.git.agx@sigxcpu.org