msm: mdss: Change DSI backlight pwm_config to use nanoseconds

The DSI panel backlight is now configured using nanosecond duty cycle
and period for pwm. A nanosecond duty cycle is needed in order to satisfy
the granularity needed for 255 backlight levels and the DSI period. If the
pwm_period is very large, then a microsecond configuration is used.

Change-Id: I9551f52360f9d5da47ab7e2da6e039ad6f2695fb
Signed-off-by: Benet Clark <benetc@codeaurora.org>