drm/i915/pmu: add pmu_to_i915() helper

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

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/20231023150256.438331-1-jani.nikula@intel.com