drm/i915/skl: Ensure pipes with changed wms get added to the state

If we're enabling a pipe, we'll need to modify the watermarks on all
active planes. Since those planes won't be added to the state on
their own, we need to add them ourselves.

Signed-off-by: Lyude <cpaul@redhat.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Cc: stable@vger.kernel.org
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471463761-26796-6-git-send-email-cpaul@redhat.com
(cherry picked from commit 05a76d3d

)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>