msm: mdss: allow re-queue of writeback buffers

When a buffer has been properly processed, client may choose to use sync
fence to acknowledge when buffer is complete. In such cases dequeue is
not necessary, allow this use case by allowing client to re-queue
writeback buffers as long as the buffer has been properly processed
already.

Change-Id: Ibac1eb9710477734e1ee2daf8136085d4439b0d4
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>