drm/i915/pmu: rearrange hrtimer pointer chasing

Do the logical step of first getting from struct hrtimer to struct
i915_pmu, and then from struct i915_pmu to struct drm_i915_private,
instead of hrtimer->i915->pmu.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231023150256.438331-3-jani.nikula@intel.com