drm: Add drm_universal_plane_init()

Add a new plane initialization interface for universal plane support
that allows a specific plane type (primary, cursor, or overlay) to
be specified.

drm_plane_init() remains as a compatibility API to reduce churn in
existing drivers.  The 'bool priv' parameter has been changed to
'bool is_primary' under the assumption that all existing uses of
private planes were representing primary planes.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>