SKL Uses the same hardware for all planes now, so called "universal"
planes. Ie both the primary planes and sprite planes share the same
logic. This patch implements the drm_plane vfuncs for "sprites" ie
planes that aren't the primary plane.

v2: Couple of fixes:
  - Actually enabled the planes and fix the plane number
Reviewed-by: NRodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: NDamien Lespiau <damien.lespiau@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>