drm: cleanup: remove drm_modeset_(un)lock_all()

Functions drm_modeset_lock_all() and drm_modeset_unlock_all() are no
longer used anywhere and can be removed.

Signed-off-by: Fernando Ramos <greenfoo@u92.eu>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210924064324.229457-17-greenfoo@u92.eu