msm: mdss: Protect ad_bl_level using ad_lock mutex

Variable ad_bl_level is used only by AD functions now,
and can be protected with ad_lock alone. Earlier it
was used outside of AD functions as well and protected
by bl_lock. Using bl_lock causing synchonization delays
with display thread hence we now use ad_lock.

Change-Id: I2ec98236c3fedee1eb4d8f8a851256ab66ab3026
Signed-off-by: Krishna Chaitanya Parimi <cparimi@codeaurora.org>