msm: mdss: Change eDP backlight pwm_config to use nanoseconds

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

Change-Id: I946e6291135f79887bfcc60fb339c4d49c1de11b
Signed-off-by: Benet Clark <benetc@codeaurora.org>