nir: Fix wrong sign in lower_rcp

nir: Fix wrong sign in lower_rcp

The nested fma calls were supposed to implement

x_new = x + x * (1 - x*src),

but instead current code is equivalent to

x_new = x - x * (1 - x*src).

The result is that Newton-Raphson steps don't improve precision at all.
This patch fixes this problem.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110435
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>