drm/i915/perf: Store shortcut to intel_uncore

Now that we have the engine stored in i915_perf, we have a means of
accessing intel_gt should we require it. However, we are currently only
using the intel_gt to find the right intel_uncore, so replace our
i915_perf.gt pointer with the more useful i915_perf.uncore.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191010150520.26488-2-chris@chris-wilson.co.uk