drm/tinydrm: Remove tinydrm_shutdown()

It's just a wrapper around drm_atomic_helper_shutdown() now.
Also store drm_device in the drvdata field, since that's what's used.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190210131039.52664-7-noralf@tronnes.org