backlight: lp855x: add support for backlight curve

backlight: lp855x: add support for backlight curve

By default, backlight updates are linear with respect
to brightness. In some cases it's desirable to use
specific transfer functions instead of a simpler linear
response.
This change adds bl_curve to the LP855x platform data,
a LUT used to implement backlight transfer functions.
LUT data is parsed from DT.

Bug 1647733

Change-Id: I0448de936149b32ade29c1a183a8ead925b617c1
Signed-off-by: Daniel Solomon <daniels@nvidia.com>
Reviewed-on: http://git-master/r/745147
(cherry picked from commit 0ddd6e1e68c30940b6d9c495eced21af3e753382)
Reviewed-on: http://git-master/r/751051
Reviewed-by: Mitch Luban <mluban@nvidia.com>