mdss: dsi: replace deprecated pwm_config_us with pwm_config

Replace deprecated PWM API pwm_config_us with pwm_config to avoid
failures while setting backlight to 0.

Change-Id: I3ab9d12572578a225cf842960d08c496891e2f13
Signed-off-by: Vishnuvardhan Prodduturi <vproddut@codeaurora.org>