nine: fixed problems with using more buffers than available

Hardcoded value 32 worked on nv50+ and r600, but on radeonsi or
others with <= 16 failed and asserted in debug mode.

Now it using value from driver, limited to 16 (not sure if it's right).

Reported-and-tested-by: Nick Sarnie (sarnex) on IRC
Signed-off-by: David Heidelberger <david.heidelberger@ixit.cz>