tests/perf: rework oa-exponent test

New issues that were discovered while making the tests work on Gen8+ :

 - we need to measure timings between periodic reports and discard all
   other kind of reports

 - it seems periodicity of the reports can be affected outside of RC6
   (frequency change), we can detect this by looking at the amount of
   clock cycles per timestamp deltas

v2: Drop some unused variables (Matthew)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>

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