drm/msm/sde: Use kthread API to initialize idle delay work

Prevent initializing delayed work during variable refresh
rate trigger. Avoid setting kthread kernel struct variable
directly. Use kthread API instead.

Change-Id: Iac3865ead22fbb2278c4da2df144d353da279d99
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>