drm/i915/pmu: Make more struct i915_pmu centric

Just tidy the code a bit by removing a sea of overly verbose i915->pmu.*.
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/20190801162330.2729-1-tvrtko.ursulin@linux.intel.com