drm/i915/pmu: Skip sampling engines if gt is asleep

If the whole GT is asleep, we know that each engine must also be asleep
and so we can quickly return without checking them all.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191218000756.3475668-1-chris@chris-wilson.co.uk