drm/dsi: turn off the wled backlight with panel power off

LAB/IBB regulator subsequent enable is failing when wled
backlight is left on with panel power off sequence. This
blocks the back to back atomic commit operation with device
open and close. Current change turns off the wled backlight
during panel power off sequence to unblock the back to back
atomic commit operation.

Change-Id: I47fb72be00fe8c1ac2234c04c58183de0e84e6be
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>