video: mdss: Sunlight readability enhancement support

 * This hardware supports a postprocessing mode which improves
   readbility in bright sunlight. Add support for enabling
   this mode when the backlight value crosses a threshold.
 * Enable the feature using /sys/class/graphics/fb0/sre
 * Also add a max backlight level for using CABC. It's not
   effective at full brightness and causes too much visible
   distortion with many images.

Change-Id: I213eb5d91e526c80a0dc88e6e05e6d6f35841fdf