msm: mdss: Update ad_bl_level on first set backlight call

AD stores a backlight level in a variable for its processing
called ad_bl_level. This needs to be initialized to the actual
backlight level at the start.

Missing this initialization and doing the backlight calculation
for AD will cause an error and ad_bl_level will never be
updated to a non zero value. This change makes sure the
ad_bl_level is set to a non zero value in all cases.

Change-Id: I285332613ffcc1d6c94247b7c9bd89f054c80d4c
Signed-off-by: Ping Li <pingli@codeaurora.org>