freedreno: make fd_batch a one-shot thing

freedreno: make fd_batch a one-shot thing

Re-allocate rather than re-use.  Originally we had an unnecessarily
complex design to avoid re-allocating cmdstream buffers.  But now that
support for "growable" cmdstream buffers has been in place for a couple
years, I guess we can care a bit less about the extra overhead on older
kernels.

But making the batches one-shot removes a class of potential race
conditions vs the flush_queue.

Signed-off-by: Rob Clark <robdclark@gmail.com>