display: exposed gamma parameters to sysfs interface

gamma values can be read inside this directory
/sys/devices/platform/mipi_orise.2049

NOTE: I have commented out/disabled modifying gamma values for now. Nexus 4 kernel uses some checksum to ensure the value doesn't go beyond a certain range, but I don't know if it applies for the Oppo panel as well

Merged following commits from faux123
* 8dabc45 - gamma control: WIP expose gamma parameters to sysfs interface (3 months ago)