pan/bi: Don't assign slots for the blend second source

Another instruction might write to the second source, and then an
INSTR_INVALID_ENC fault will be raised because the tuple will write to
and read from the register at the same time.

Fixes: 795638767d ("pan/bi: Use fused dual source blending")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15250>

src/panfrost/bifrost/bi_pack.c

@@ -120,6 +120,12 @@ bi_assign_slots(bi_tuple *now, bi_tuple *prev)
 
         if (now->add) {
                 bi_foreach_src(now->add, src) {
+                        /* This is not a real source, we shouldn't assign a
+                         * slot for it.
+                         */
+                        if (now->add->op == BI_OPCODE_BLEND && src == 4)
+                                continue;
+
                         if (!(src == 0 && read_dreg))
                                 bi_assign_slot_read(&now->regs, (now->add)->src[src]);
                 }