drm/atomic-helpers: Export drm_atomic_helper_update_legacy_modeset_state

This is useful for drivers which have their own modeset infrastructure
but want to reuse most of the legacy state frobbery from the helpers.
i915 wants this.

v2: Add header declaration.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>