drm/plane-helper: unexport drm_primary_helper_create_plane

We shouldn't tempt driver writers into using this since it uses a
default format list which is likely wrong. And when that's done we can
simplify the code a bit, too.

Noticed while reviewing a patch from Laurent.

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>