drm/i915: Use correct dpms for intel_enable_crt.

With the conversion to atomic only on/off are still supported.
The rest is mapped to one of those, and when enable is called
DPMS_ON should be true.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1455697119-31416-5-git-send-email-maarten.lankhorst@linux.intel.com