armsoc_gc: Fix errors when chaining up ValidateGC
armsoc_gc: Fix errors when chaining up ValidateGC

This became "const GCFuncs *" in modern X servers, which is silly, but
let's work around this a bit by modifying our copy.

Popolon/xf86-video-armsoc,commshare/xf86-video-armsoc,commshare/xf86-video-armsoc,Popolon/xf86-video-armsoc