msm: mdss: do not assign ad_calc_bl if AD offsets are absent

If AD block offsets are absent in the device tree then do not
initialize ad_calc_bl. This would prevent unnecessary function
calls into AD backlight processing functions on targets where
AD is not enabled.

Change-Id: I6257d46a8ece9db134697170b91af9a9bead3b72
Signed-off-by: Krishna Chaitanya Parimi <cparimi@codeaurora.org>