drm/i915/plane: add intel_plane_helper_add() helper

Add a small helper to keep intel_plane_helper_funcs static.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210518132426.7567-1-jani.nikula@intel.com