msm: mdss: Call wait4comp after commit for wb intf

Writeback path uses the comp event allocated on
stack to wait for ISR done. It crashes if
ISR tries to set the comp event after kickoff
thread timeout. This change removes the comp
event usage for writeback interface and starts
to use the wait4comp API like video/command
mode interfaces.

CRs-fixed: 555036
Change-Id: I74994a0cbe4039c6d3d4a51d573d343751182bff
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>