ddebug: rewrite to always use a threaded approach

This patch has multiple goals:

1. Off-load the writing of records in 'always' mode to another thread
   for performance.

2. Allow using ddebug with threaded contexts. This really forces us to
   move some of the "after_draw" handling into another thread.

3. Simplify the different modes of ddebug, both in the code and in
   the user interface, i.e. GALLIUM_DDEBUG. In particular, there's
   no 'pipelined' anymore, since we're always pipelined; and 'noflush'
   is replaced by 'flush', since we no longer flush by default.

4. Fix the fences in pipelining mode. They previously relied on writes
   via pipe_context::clear_buffer. However, on radeonsi, those could
   (quite reasonably) end up in the SDMA buffer. So we use the newly
   added PIPE_FLUSH_{TOP,BOTTOM}_OF_PIPE fences instead.

5. Improve pipelined mode overall, using the finer grained information
   provided by the new fences.

Overall, the result is that pipelined mode should be more useful, and
using ddebug in default mode is much less invasive, in the sense that
it changes the overall driver behavior less (which is kind of crucial
for a driver debugging tool).

An example of the new hang debug output:

  Gallium debugger active.
  Hang detection timeout is 1000ms.
  GPU hang detected, collecting information...

  Draw #   driver  prev BOP  TOP  BOP  dump file
  -------------------------------------------------------------
  2          YES      YES    YES  NO   /home/nha/ddebug_dumps/shader_runner_19919_00000000
  3          YES      NO     YES  NO   /home/nha/ddebug_dumps/shader_runner_19919_00000001
  4          YES      NO     YES  NO   /home/nha/ddebug_dumps/shader_runner_19919_00000002
  5          YES      NO     YES  NO   /home/nha/ddebug_dumps/shader_runner_19919_00000003

  Done.

We can see that there were almost certainly 4 draws in flight when
the hang happened: the top-of-pipe fence was signaled for all 4 draws,
the bottom-of-pipe fence for none of them. In virtually all cases,
we'd expect the first draw in the list to be at fault, but due to the
GPU parallelism, it's possible (though highly unlikely) that one of
the later draws causes a component to get stuck in a way that prevents
the earlier draws from making progress as well.

(In the above example, there were actually only 3 draws truly in flight:
the last draw is a blit that waits for the earlier draws; however, its
top-of-pipe fence is emitted before the cache flush and wait, and so
the fact that the draw hasn't truly started yet can only be seen from a
closer inspection of GPU state.)

Acked-by: Marek Olšák <[email protected]>

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