drm/i915/rpm: add rpm_to_i915() helper around container_of()

Reduce the duplication.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231205121545.2338665-1-jani.nikula@intel.com