drm/panel: s6e63m0: Implement 28 backlight levels

A later version of the s6e63m0 driver in the Samsung
GT-I9070 vendor tree provides 28 different backlight
levels making use of elaborate control of the ACL
and ELVSS regulator. Implement this more fine-grained
backlight control.
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Guido Günther <agx@sigxcpu.org>
Cc: Stephan Gerhold <stephan@gerhold.net>
Cc: Paweł Chmiel <pawel.mikolaj.chmiel@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201117175621.870085-2-linus.walleij@linaro.org