Skins validating twice because of styling code in updateDisplayList

Steps to reproduce:
1. Create a custom PanelSkin and put a trace in updateDisplayList().
2. Set borderVisible="false" on a Panel with the custom PanelSkin.
3. Rerun without setting borderVisible.

Actual Results:
updateDisplayList gets called twice when borderVisible="false".

Expected Results:
updateDisplayList should only get called once.

Workaround (if any):
Reskin and take out styling code.

It looks like sometimes our styling code causes extra invalidations to the skin. We should investigate which styles and which skins have this issue and perhaps move the code to styleChanged.