msm: mdss: call wait4comp before wb kickoff

when wfd is tear down, HW still fetch the address of buffer queue
which was already released and while cancelling the buffer cause the
iommu page fault. Call wait4comp before kickoff is called to avoid
iommu page fault.

Change-Id: Ia597b395e13072374cfe3d5fa8216363946bff16
Signed-off-by: Raghavendra Ambadas <rambad@codeaurora.org>