drm/i915/selftests: Fix GT PM sort comparators

drm/i915/selftests: Fix GT PM sort comparators

Compare the sampled clock values instead of their addresses. Comparing
addresses leaves the samples unsorted, preventing the code from discarding
the minimum and maximum samples.

Fixes: 1a5392479207 ("drm/i915/selftests: Measure CS_TIMESTAMP")
Signed-off-by: Emre Cecanpunar <emreleno@gmail.com>
Signed-off-by: Tvrtko Ursulin <tursulin@ursulin.net>
Link: https://lore.kernel.org/r/20260714220430.238433-1-emreleno@gmail.com
(cherry picked from commit 682ea2d28d18bb06f9fc663cb5ab7e80dc0e606a)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>