msm: mdss: Prevent ALS data configuration when AD is disabled

If assertive display is disabled, backlight data structure is reset
to NULL, which in some conditons gets dereferenced or casuses
crash.

Change-Id: I535eac6d2f0aef222fc2846f80d72db1f3e3480d
Signed-off-by: Ping Li <quicpingli@codeaurora.org>