Only force camera above terrain when loading is finished.

Fixes #6962

I'm not sure if we introduced a regression somewhere or if we just never
noticed, but the desired camera behavior is that it not force the view to
be above terrain if there is still terrain loading.  I also confirmed
that #4075 is still fixed even with this change.

Source/Scene/Camera.js

@@ -958,7 +958,10 @@ define([
         if (this._suspendTerrainAdjustment) {
             this._suspendTerrainAdjustment = !globeFinishedUpdating;
         }
-        this._adjustHeightForTerrain();
+
+        if (globeFinishedUpdating) {
+            this._adjustHeightForTerrain();
+        }
     };
 
     var setTransformPosition = new Cartesian3();