i965: perf: ensure isolated timer reports while idle don't confuse filtering

i965: perf: ensure isolated timer reports while idle don't confuse filtering

From experimentation in IGT, we found that the OA unit might label
some report as "idle" (using an invalid context ID), right after a
report for a given context. Deltas generated by those reports actually
belong to the previous context, even though they're not labelled as
such.

This change makes ensure that while reading OA reports, we only
consider the GPU actually idle after 2 reports with an invalid context
ID.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>