drm/atomic-helper: Fix kerneldoc for prepare_planes

Copypaste-fail from cleanup_planes. Reported by Tvrtko.

Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Reviewed-by: NTvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@intel.com>

drivers/gpu/drm/drm_atomic_helper.c

@@ -1096,9 +1096,9 @@ EXPORT_SYMBOL(drm_atomic_helper_commit);
  */
 
 /**
- * drm_atomic_helper_prepare_planes - prepare plane resources after commit
+ * drm_atomic_helper_prepare_planes - prepare plane resources before commit
  * @dev: DRM device
- * @state: atomic state object with old state structures
+ * @state: atomic state object with new state structures
  *
  * This function prepares plane state, specifically framebuffers, for the new
  * configuration. If any failure is encountered this function will call