drm/i915/pmu: Convert sampling to gt

Engines and frequencies are a GT thing so adjust sampling routines to
match.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190801162330.2729-3-tvrtko.ursulin@linux.intel.com