drm/i915/skl: Adjust assert_sprites_disabled()

Let's put to good use the new PLANE_CTL macros.

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>