drm/msm/sde: Prevent kthread off work initialization during dms

Kthread off work should be initialized only during resume, so add
necessary checks to avoid initialization during other cases.

Change-Id: Ic4f9935465481a06c27f7fbdaa81842369f7e3dc
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>