drm/i915: update dpms property in set_mode

Hopefully this makes userspace slightly less confused about us
frobbing the dpms state behind its back. Yeah, it would be better
to be more careful with not changing the dpms state, but that is
quite more invasive.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>