drm/rockchip: remove unused psr_list{,_lock}

Some leftover cleanup from commit 6c836d96 ("drm/rockchip: Use the
helpers for PSR").

Cc: Heiko Stuebner <heiko@sntech.de>
Cc: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210915135007.1.I926ef5cef287047c35a17e363c919599c6ee6e4c@changeid