BACKLIGHT CONTROL: Allow user to set min backlight when auto mode.

By user request, i have made simple overide interface for min backlight.

Normally ROM min backlight is set to 86 and max is 255.

When screen is dimmed before turn off it's 50.

So now user can ask kernel to set min backlight in dark from 30 to 89

and by this reduce the MIN backlight value.