Add a plane type property to allow userspace to distinguish plane types.

v2: Driver-specific churn eliminated now that drm_plane_init() and
    drm_universal_plane_init() were separated out in a previous patch.
Signed-off-by: NMatt Roper <matthew.d.roper@intel.com>
Reviewed-by: NRob Clark <robdclark@gmail.com>