drm/i915/pmu: add event_to_pmu() helper

It's tedious to duplicate the container_of() everywhere. Add a helper.

Also do the logical steps of first getting from struct perf_event to
struct i915_pmu, and then from struct i915_pmu to struct
drm_i915_private if needed, instead of perf_event->i915->pmu. Not all
places even need the i915 pointer.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231023150256.438331-2-jani.nikula@intel.com