Support for display gamma control

Originally written by Paul Reioux (faux123)

Modified from his original implementation as follows:
* Remove kgamma_ctrl sysfs entry - it did absolutely nothing
* Alter the show() functions to output the checksum in the
first entry, in order to match the expected input of
the store() functions.  This should not break existing
applications
* Move store/show code to a common function
* Add limit checks from Mike Cain (motley)
* Added a kgamma_apply sysfs entry which will apply
  the current state when anything is stored to it,
  instead of waiting until the next time the LCD
  is power cycled

Change-Id: I8835e42709cefa547e02e28a7e5a381d88282700