mdss: enable content adaptive backlight control (cabc) for find7 variants

the original vendor code allows for PWM control of backlight in its own
mdss driver but for some reason the cabc control was hardcoded to use level
3 only for HW revision > 20 (find7 variants).

Re-enble the cabc and pwm backlight control for find7 variants and clean up
the coding style to conform to Linux kernel coding standards as well.

Change-Id: I8bd2fe2d595edd67dedd7e536225fbfc7d5ca3a9
Signed-off-by: Paul Reioux <reioux@gmail.com>