drm/msm/dpu: use msm wq for idle power collapse

msm is using msm wq for dispatching commit and vblank
events. Switch idle power collapse feature also to use
msm wq to handle delayed work handlers so that
msm can get rid of redundant display threads.

changes in v2:
	- patch introduced in v2
changes in v3:
	- none
changes in v4:
	- use msm wq for delayed works
changes in v5:
	- none

Signed-off-by: Jeykumar Sankaran <jsanka@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>