freedreno: don't overflow cmdstream buffer so much

freedreno: don't overflow cmdstream buffer so much

We currently aren't too clever about dealing with running out of
cmdstream buffer space.  Since we use a single buffer for both drawing
and tiling commands, we need to ensure there is enough space at the tail
of the cmdstream buffer to fit the tiling commands.

Until we get more clever, the easy solution is a threshold to trigger
flushing rendering even if the application does not trigger flush (swap,
changing render target, etc).  This way we at least don't crash for apps
that do several thousand draw calls (like some piglit tests do).

Signed-off-by: Rob Clark <robclark@freedesktop.org>