freedreno: threaded batch flush

freedreno: threaded batch flush

With the state accessed from GMEM+submit factored out of fd_context and
into fd_batch, now it is possible to punt this off to a helper thread.
And more importantly, since there are cases where one context might
force the batch-cache to flush another context's batches (ie. when there
are too many in-flight batches), using a per-context helper thread keeps
various different flushes for a given context serialized.

TODO as with batch-cache, there are a few places where we'll need a
mutex to protect critical sections, which is completely missing at the
moment.

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