Ville ocd'ed the parameter name, but forgot to update the docs!

Fixes: df86af91 ("drm/plane-helper: Add drm_plane_helper_check_state()")
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471034937-651-21-git-send-email-daniel.vetter@ffwll.ch