drm/i915/pmu: Skip sampling engines with no enabled counters

As we have more and more engines do not waste time sampling the ones no-
one is monitoring.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230519154946.3751971-4-umesh.nerlige.ramappa@intel.com