drm/panel: s6e63m0: Support max-brightness

The "max-brightness" is a standard backlight property that
we need to support for the Samsung GT-I8190 Golden because
the display will go black if we crank up the brightness
too high.

As the platform needs this ability to give picture this is
a regression fix along with the addition of the property
to the GT-I8190 device tree.

Cc: Stephan Gerhold <stephan@gerhold.net>
Fixes: 9c3f0a0d ("drm/panel: s6e63m0: Implement 28 backlight levels")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201214222210.238081-1-linus.walleij@linaro.org