mistral: Disable global buffers that are currently broken

Signed-off-by: gatecat <gatecat@ds0.me>

mistral/globals.cc

@@ -26,6 +26,8 @@ NEXTPNR_NAMESPACE_BEGIN
 void Arch::create_clkbuf(int x, int y)
 {
     for (int z = 0; z < 4; z++) {
+    	if (z != 2)
+    		continue; // TODO: why do other Zs not work?
         // For now we only consider the input path from general routing, other inputs like dedicated clock pins are
         // still a TODO
         BelId bel = add_bel(x, y, id(stringf("CLKBUF[%d]", z)), id_MISTRAL_CLKENA);