msm: mdss: Update condition check for AD BL attenuation calculation

Enable the AD backlight attenuation calculation every time the
mdss_fb_set_backlight function is called, so that the backlight
sent to panel can be properly attenuated if AD is enabled.

Change-Id: If751b69fe5f927c15580332645e7e26fe6cb778c
Signed-off-by: Ping Li <quicpingli@codeaurora.org>