Remove incorrect isEnabled() check

 Bug: 15683945

 invokeFunctor() does not require isEnabled() to be true
 to execute. Specifically it does not care if HardwareRenderer
 has a Surface or not, as it will fall back to a pbuffer surface
 if available. If there is no context at all, it will invoke
 with kModeProcessNoContext.

Change-Id: Ie6abe566b92c8fa33bb7a673fc3b9fa9ce913446