msm: display: reduce display commit prepare waiting time

Unblock overlay_set, prepare and buffer sync ioctls when
commit thread finishes hardware programming.
Use wait_for_kickoff instead of pan_idle for these ioctls.

CRs-fixed: 620740
Change-Id: Iefb6cbc390d2130926fdcb93688b4d92bfb26937
Signed-off-by: Ken Zhang <kenz@codeaurora.org>