Cleaning up references to mDragView after reordering Workspaces

Change-Id: I480142b4714034c5579232a6a0d30f3aecb687ac
Fix NPE if a moved cell does not have a parent

This NPE should actually never happen. It did however and crashed
Trebuchet.
Symptom that will lead to this NPE: Drag Shortcut from homescreen
to somewhere else. Old icon will not get removed while dragging.
This should also be fixed!

Change-Id: I5b5070c93077e69f0f63a26d337fb9aee83ec6d2
Merge branch 'cm-11.0' of https://github.com/CyanogenMod/android_packages_apps_Trebuchet into cm-11.0