video: mdss: Set initial backlight after first frame done

 * Garbage is randomly appearing during unsuspend on the Truly panel.
 * Userspace is updating the backlight sysfs very quickly (good), but in
   some cases there is random garbage in frame memory which might
   end up being displayed to the user (bad).
 * Update the backlight after the first frame done in this case
   to make sure any random crap left in the frame memory is overwritten
   by a proper image.

Change-Id: I800ce2bd2f428b09b0dbf3c5968f92e3acb60f49