drm/i915: resume MST after reading back hw state

Otherwise the MST resume paths can hit DPMS paths
which hit state checker paths, which hit WARN_ON,
because the state checker is inconsistent with the
hw.

This fixes a bunch of WARN_ON's on resume after
undocking.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: stable@vger.kernel.org
Signed-off-by: Jani Nikula <jani.nikula@intel.com>