msm: mdss: Remove pointless NULL check in pp_ad_calc_bl

Clang warns:

../drivers/video/fbdev/msm/mdss_mdp_pp.c:3372:8: warning: address of
array 'ad->bl_att_lut' will always evaluate to 'true'
[-Wpointer-bool-conversion]
                !ad->bl_att_lut) {
                ~~~~~^~~~~~~~~~

Fixes: eeb8889e ("fbdev: msm: Add snapshot of display framebuffer driver")
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Change-Id: Ib40fb1946568fac1922dcb82c1b5d8aa269bfd04
Signed-off-by: improbir <ghosh_probir@yahoo.com>