msm_fb: call the backlight control before FB_BLANK_POWERDOWN

You might see the white line while lcd off. You should turn lcd backlight
off before lcd off to avoid that issue. So call the notifier for backlight
before handling of FB_BLANK_POWERDOWN in msm_fb.

Change-Id: I3bd08025a949ec02a1fba7cef98a13d8977c135b