drm/i915: Use crtc->hwmode for vblanks, v2.

intel_crtc->config will be removed eventually, so use crtc->hwmode.
drm_atomic_helper_update_legacy_modeset_state updates hwmode,
but crtc->active will eventually be gone too. Set dotclock to zero
to indicate the crtc is inactive.

Changes since v1:
- With the hwmode update in drm*update_legacy_modeset_state removed,
  intel_modeset_update_state has to assign it instead.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>