msm: mdss: Reset AD event notification count

Reset the AD event notification count whenever ALS or backlight
input of AD has been made. This provides a way for user space to
track and sync with the AD sync iteration numbers in driver.

Change-Id: I4e4a2a93378c6abacb4ffbcf8f4a14aaf38789af
Signed-off-by: Ping Li <pingli@codeaurora.org>