drm/shmobile: Restrict plane loops to only operate on legacy planes

Ensure that existing driver loops over all planes do not change behavior
when we begin adding new types of planes (primary and cursor) to the DRM
plane list in future patches.

Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>