drm/msm/sde: queue pp work items to dedicated event thread

Histogram and AD pp feature works are queued in the interrupt context
to the same crtc_event thread as others like vblank work items.
This is causing delay in fence release as work items are serialized
which is further leading to performance issue. This change will allow
the works to run parallel in different dedicated event thread.

Change-Id: I040d1a446dc6644609c2ee31b051073245da1bba
Signed-off-by: Raviteja Tamatam <travitej@codeaurora.org>