nvc0: increase NOUVEAU_TRANSFER_PUSHBUF_THRESHOLD to 1024 on Kepler+

nvc0: increase NOUVEAU_TRANSFER_PUSHBUF_THRESHOLD to 1024 on Kepler+

Gives a +3.89% to +5.27% FPS improvement with Hitman and +2.73% to +2.82%
FPS improvement with Dirt Rally on my GTX 1060.

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>