msm: mdss: dsi: set backlight handle to NULL in error case

Set pwm backlight handle to NULL during dsi panel
pwm configuration when it fails with an error.

Change-Id: Ia28ab3728888731999c61672faf1bd2f5be1965f
Signed-off-by: Chirag Khurana <ckhurana@codeaurora.org>
Signed-off-by: Nirmal Abraham <nabrah@codeaurora.org>