nv50/ir: don't emit src2 in immediate form
In the immediate form, src2 == dst, so it does not need to be emitted. Otherwise it overlaps with the immediate value's low bits. Fixes: 09ee9072 (nv50/ir: Fold IMM into MAD) Cc: "10.6" <mesa-stable@lists.freedesktop.org> Signed-off-by:Ilia Mirkin <imirkin@alum.mit.edu> (cherry picked from commit c3215ef2)