Apologies for slipping to a private thread, this was unintended, I wanted to thank Jani Nikula for helping out greatly and confirming that, in fact, the GTDRIVER_MAILBOX_DATA* registers are the same as their GEN6_PCODE_DATA* counterparts as defined in the Linux kernel(see first message in the thread).
Thanks again to Jani Nikula for the assistance, this resolves the issue.
