drm/i915/skl: Implement drm_plane vfuncs

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: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>