msm: mdss: Fix the AD backlight attenuation implementation

Fix the AD backlight attenuation implementation by sending the
attenuation backlight to both panel and AD HW block, to achieve power
saving.

Change-Id: I64c79d7aa439a8d9e722b4b098c0b163fd115699
Signed-off-by: Ping Li <quicpingli@codeaurora.org>