i915/perf: Fix CS timestamp vs OA timstamp mismatch

CS timestamp can have an additional configuration that shifts the
timestamp value by a specified number of bits. On DG2, this additional
configuration is ignored by OA, so OA timestamps are missing the shift.

Add newly defined interface to get the exact OA timestamp frequency so
that the deltas can be accurately calculated.

Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>