i965: Clear some undefined fields of g0 when using them for gen6 FB writes.

This doesn't appear to help any testcases I'm looking at, but it looks
like it's required.

src/mesa/drivers/dri/i965/brw_fs.cpp

@@ -1961,6 +1961,20 @@ fs_visitor::generate_fb_write(fs_inst *inst)
 	 brw_MOV(p,
 		 brw_message_reg(inst->base_mrf),
 		 brw_vec8_grf(0, 0));
+
+	 if (inst->target > 0) {
+	    /* Set the render target index for choosing BLEND_STATE. */
+	    brw_MOV(p, retype(brw_vec1_reg(BRW_MESSAGE_REGISTER_FILE, 0, 2),
+			      BRW_REGISTER_TYPE_UD),
+		    brw_imm_ud(inst->target));
+	 }
+
+	 /* Clear viewport index, render target array index. */
+	 brw_AND(p, retype(brw_vec1_reg(BRW_MESSAGE_REGISTER_FILE, 0, 0),
+			   BRW_REGISTER_TYPE_UD),
+		 retype(brw_vec1_grf(0, 0), BRW_REGISTER_TYPE_UD),
+		 brw_imm_ud(0xf7ff));
+
 	 implied_header = brw_null_reg();
       } else {
 	 implied_header = retype(brw_vec8_grf(0, 0), BRW_REGISTER_TYPE_UW);