Partly revert [4f54b0f229b5] and move draining to m5.changeToTiming

Changeset 4f54b0f229b5 removed the call to doDrain in changeToTiming
based on the assumption that the system does not need draining when
running in atomic mode. This is a false assumption since at least the
System class requires the system to be drained before it allows
switching of memory modes. This patch reverts that part of the
changeset.

src/python/m5/simulate.py

@@ -207,6 +207,7 @@ def changeToTiming(system):
               (type(system), objects.Root, objects.System)
 
     if system.getMemoryMode() != objects.params.timing:
+        doDrain(system)
         print "Changing memory mode to timing"
         system.setMemoryMode(objects.params.timing)