test/perf: fixup inverted condition

The previous patch said :

"verify that the time is always longer or equal to the period we've
 asked for"

This is an obvious error, it only worked on my machine and the CI
because only one longer period was observed. But another CI run caught
the issue :

  https://intel-gfx-ci.01.org/tree/drm-tip/IGT_4280/shard-glkb6/igt@perf@oa-exponents.html

Fixes: c3d11ca104fa ("tests/perf: make oa-exponents subtest more reliable")
Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
1 file changed