drm: replace drm_modeset_lock_all() in drm_client_modeset_dpms_legacy()

This patch helps complete Use  DRM_MODESET_LOCK_ALL* helpers instead of
boilerplate todo in Documentation/gpu/todo.rst

Signed-off-by: Joseph Schulte <joeschulte.js@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210125144815.8389-1-joeschulte.js@gmail.com