drm/todo: i915 could use device_link_add

Noticed while reading some unrelated patches. Unfortunately Imre's
patch to add our early/late hooks predated the device_link
infrastructure by 2 years.

Cc: Imre Deak <imre.deak@intel.com>
Cc: Takashi Iwai <tiwai@suse.de>
Acked-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180220132017.30719-1-daniel.vetter@ffwll.ch