drm/tinydrm: Remove tinydrm_device

No more users left so it can go alongside its helpers.
Update the tinydrm docs description and remove todo entry.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190225144232.20761-7-noralf@tronnes.org