msm: mdss: Correctly update pwm configuration

Backlight pwm was incorrectly updated while changing the backlight
level. We need to disable the pwm configuration before updating
it with the updated value.

Change-Id: Ie027c26a614c6bb0ffdc51218cf4c81b26ceb736
Signed-off-by: Siddhartha Agrawal <agrawals@codeaurora.org>