Update Flickable velocity/overbound for each axis independently

Update Flickable velocity/overbound for each axis independently

We were attempting to update velocities/overbound correction for both
axes even if only one of them had changed, leading to inaccurate values
for one of the axes.  Also removes a bunch of code duplication.

Change-Id: I6a50b908992c976889373f541bdd1abad462e247
Reviewed-by: Bea Lam <bea.lam@nokia.com>