RFR(T): 8230327: Make G1DirtyCardQueueSet free-id init unconditional

[Kim Barrett]

Please review this trivial cleanup of G1DirtyCardQueueSet
initialization. With the separation of G1RedirtyCardsQueueSet from
G1DirtyCardQueueSet, the latter is now a singleton class that should
always have an associated G1FreeIdSet.  We can now unconditionally
construct that object when constructing the DCQS.

CR:
https://bugs.openjdk.java.net/browse/JDK-8230327

Webrev:
http://cr.openjdk.java.net/~kbarrett/8230327/open.00/

Testing:
Local (linux-x64) hotspot:tier1.