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: NRodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: NDamien Lespiau <damien.lespiau@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_dma.c

@@ -1534,7 +1534,7 @@ static void intel_device_info_runtime_init(struct drm_device *dev)
 
 	info = (struct intel_device_info *)&dev_priv->info;
 
-	if (IS_VALLEYVIEW(dev))
+	if (IS_VALLEYVIEW(dev) || INTEL_INFO(dev)->gen == 9)
 		for_each_pipe(dev_priv, pipe)
 			info->num_sprites[pipe] = 2;
 	else