drm/i915: Set crtc DPMS mode to ON in intel_crtc_mode_set

This corrects the DPMS mode tracking so that the DPMS code will
actually turn the CRTC off the next time the screen saves.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>