Tuna : Add display gamma calibration

Allow user to calibrate display gamma.

PS 3: Cleanup code.
      Better gamma values
      Needs: http://review.cyanogenmod.org/48901

PS 4: Restore original commit message

PS 5: Restore old ordering

PS 7: Restore original names of variables & unify both files

Change-Id: I2aa1d746ec6c8372f80e022b786078c5ded68612