nak/sm20: Fix the encoding of fset

We weren't setting the accum src (which should be pT for now) and we put
ftz in the wrong place.  Bit 5 is actually .bf which we want to set all
the time in order to get a float output.  (Otherwise it gives an integer
output.)

Fixes: a3330f1d46 ("nak/sm20: Add float ops")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35170>

src/nouveau/compiler/nak/sm20.rs

@@ -812,12 +812,14 @@ impl SM20Op for OpFSet {
             None,
         );
 
-        e.set_bit(5, self.ftz);
+        e.set_bit(5, true); // .bf
         e.set_bit(6, self.srcs[1].src_mod.has_fabs());
         e.set_bit(7, self.srcs[0].src_mod.has_fabs());
         e.set_bit(8, self.srcs[1].src_mod.has_fneg());
         e.set_bit(9, self.srcs[0].src_mod.has_fneg());
+        e.set_pred_src(49..53, &SrcRef::True.into());
         e.set_float_cmp_op(55..59, self.cmp_op);
+        e.set_bit(59, self.ftz);
     }
 }