Replace hardcoded number with a constexpr

Change-Id: Iffd45a942d23013696213d6d40587c832caf2879
diff --git a/av1/qmode_rc/ratectrl_qmode.cc b/av1/qmode_rc/ratectrl_qmode.cc
index ed36d47..af01a18 100644
--- a/av1/qmode_rc/ratectrl_qmode.cc
+++ b/av1/qmode_rc/ratectrl_qmode.cc
@@ -1556,7 +1556,7 @@
       if (gop_frame.update_type == GopFrameType::kRegularGolden ||
           gop_frame.update_type == GopFrameType::kRegularKey ||
           gop_frame.update_type == GopFrameType::kRegularArf) {
-        param.q_index = 5;
+        param.q_index = kSecondTplPassQp;
         param.rdmult = av1_compute_rd_mult_based_on_qindex(
             AOM_BITS_8, ARF_UPDATE, param.q_index);
       }
diff --git a/av1/qmode_rc/ratectrl_qmode.h b/av1/qmode_rc/ratectrl_qmode.h
index 375305f..86e15b0 100644
--- a/av1/qmode_rc/ratectrl_qmode.h
+++ b/av1/qmode_rc/ratectrl_qmode.h
@@ -26,6 +26,7 @@
 constexpr int kMinIntervalToAddArf = 3;
 constexpr int kMinArfInterval = (kMinIntervalToAddArf + 1) / 2;
 constexpr double kIntArfAdjFactor = 0.5;
+constexpr int kSecondTplPassQp = 5;
 
 struct TplUnitDepStats {
   double propagation_cost;