drm/i915: init sprites with univeral plane init function

Really just for completeness - old init function ends up making the plane
exactly the same way due to the way the enums are set up.

Signed-off-by: Derek Foreman <derek.foreman@collabora.co.uk>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>