support active flush without attach buffers

support active flush without attach buffers

for supporting double buffer queue of compositor

1. send once wait_usable request in alloc callback.
   server send attach buffer after get wait usable request.
2. flush queue in can dequeue notify after active event.
   if queue is active flushed with active between can_dequeue
   and dequeue function, dequeue is failed.
   because queue state is deactive when can_dequeue is called,
   dequeue is called without attach buffer.

Change-Id: I85cca34831397f57870f19038838ee8fde42cf9c