drm/msm/sde: remove thread flush request for same thread

Input event work is scheduled on crtc_commit thread and
encoder disable API tries to flush the input work. This
can cause deadlock to crtc_commit thread because it may
wait for pending job on its own thread. Fix this deadlock
by removing the flush_kthread API call.

Change-Id: I39cb4249edf36cbc27bbad9726c52613e83eff0f
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>