drm/msm/sde: Update BL scale value in atomic commit

Remove the display backlight scaling logic from connector setting
property to atomic commit phase. This is to avoid backlight scaled
multiple times since the check during validate and the actual atomic
check during commit phase would both modify the backlight programming.

Change-Id: Ia268dd592878f5bb031a4c07167f1ec833366596
Signed-off-by: Xu Yang <yangxu@codeaurora.org>