Support restoring B/C OA counter config in RC6 WA BB

[rib]

So we can aim to leave RC6 enabled while capturing OA metrics, the i915 perf driver needs to support adding the the B/C counter configuration to the RC6 Workaround Batch Buffer.

Note: Gen 9 has an Errata where the RC6 WA BB is not always being executed, whereby the MUX config won't be restored. Userspace may need to be involved in recognising when the metrics configuration isn't correctly restored and request a re-configuration.