intel/perf: set read buffer len to 0 to identify empty buffer

We always add an empty buffer in the list when creating the query.
Let's set the len appropriately so that we can recognize it when we
read OA reports up to the end of a query.

We were using an 0 timestamp value associated with the empty buffer
and incorrectly assuming this was a valid value. In turn that led to
not reading enough reports and resulted in deltas added to our counter
values which should have been discarded because those would be flagged
for a different context.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Mark Janes <mark.a.janes@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
(cherry picked from commit 9d0a5c81)