r600/sfn: Make use of four clause local registers

r600/sfn: Make use of four clause local registers

The hardware is actually configures like this, but for fma64
we have to sacrifice a "normal" register to allocate z and w
channels, even though the result written there is not used.

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24638>