gui: CpuConsumer::lockNextBuffer change return code when too many bufs acquired

- Return NOT_ENOUGH_DATA instead of INVALID_OPERATION when too many
  buffers have already been locked.
- INVALID_OPERATION is nominally used when something irrecoverable happens,
  but in this case the client just needs to call unlockBuffer to go back into a
  good state.

Bug: 10333400
Change-Id: I3a034d77de85741429f832a90eedd670afa1dc94