drm/i915: do display power state notification on crtc enable/disable

The spec says to notify prior to power down and after power up. It is
unclear whether it makes a difference.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>