drm: Fix kerneldoc and remove unused struct member in self_refresh helper

Artifacts of previous revisions.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link to v1: https://patchwork.freedesktop.org/patch/msgid/20190917200443.64481-1-sean@poorly.run
Link: https://patchwork.freedesktop.org/patch/msgid/20190918200734.149876-1-sean@poorly.run

Changes in v2:
- None