drm/plane-helper: Fix transitional helper kerneldocs

drm_plane_helper_{update,disable} are not specific to primary planes;
fix some copy/paste summaries to avoid confusion.

Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>