r600: reduce num compute threads to 1024.

I copied this value from radeonsi, but it was wrong, 1024
seems to be correct answer from looking at gpuinfo.

This should fix a few compute shader related hangs. (at least in CTS)

Cc: <mesa-stable@lists.freedesktop.org>
(airlied: pushed because it avoids hangs)

(cherry picked from commit 9039cf70)