intel/perf: simplify the processing of OA reports

This is a more accurate description of what happens in processing the
OA reports.

Previously we only had a somewhat difficult to parse state machine
tracking the context ID.

What we really only need to do to decide if the delta between 2
reports (r0 & r1) should be accumulated in the query result is :

   * whether the r0 is tagged with the context ID relevant to us

   * if r0 is not tagged with our context ID and r1 is: does r0 have a
     invalid context id? If not then we're in a case where i915 has
     resubmitted the same context for execution through the execlist
     submission port

v2: Update comment (Ken)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
(cherry picked from commit 8c0b0582)