drm/i915/skl: Skylake has 2 "sprite" planes per pipe

On Skylake, we use plane1 as primary plane and plane2/3 as sprite
planes.

v2: Rebase on top of the for_each_pipe() change adding dev_priv as first
    argument.

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>