drm/i915: use pointer to i915 instead of rpm in wakeref

Currently a pointer to an intel_runtime_pm structure is stored in the
wake reference structures so the runtime data can be accessed.  We can
save the entire device information (drm_i915_private) instead, since
we'll need to reference the new workqueue we'll add in subsequent
patches.

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ec0eb5149120d04f3d9870d7671ef10103e6fc29.1686231190.git.jani.nikula@intel.com