msm: mdss: use new PWM API to replace the old one

Because there are more than two pwm, this need to chose the right channel,
but the old API can not treat this, so use the new API to replace it.

Change-Id: I52d40ddd2fe85dc07924e8139087f5993fad2a56
Signed-off-by: Wenjun Zhang <wjzhan@codeaurora.org>