msm: mdss: make sure backlight is off during suspend

The backlight of the display panel will not get turned off during
suspend if there are no display commits between suspend/resume
since allow_bl_update will only be set to 'true' after first
valid display commit after resume. This is possible for power
off charging use-case where there will be only one display
commit initially and later just the content of the buffer gets
updated without anymore display commits across suspend/resume.
Make sure the backlight is turned off during suspend by
explicitly setting allow_bl_update to 'true' before trying to
set the backlight to zero.

Change-Id: I10f39e3913e5b40a61066f0bbce3a7a8f1a9441b
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>