drm/i915: Update missing kerneldoc

Not sure why so much slips through when 0day is catching these. Hopefully
the much faster sphinx toolchain helps in unlazying people.

Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1468612088-9721-10-git-send-email-daniel.vetter@ffwll.ch