msm: mdss: Display gamma control

Default Nexus 5 display gamma calibration is OK at best. Enable
the user to tune RGB colors and white point according to his/her
preferences.

sysfs is located under: /sys/module/dsi_panel/*

Change-Id: If80b06dd457efddf7a6ac21d704036a763968168

Conflicts:
	drivers/video/msm/mdss/mdss_dsi_panel.c