doc: drm: remove TODO entry regarding DRM_MODSET_LOCK_ALL cleanup

The previous commits do exactly what this entry in the TODO file asks
for, thus we can remove it now as it is no longer applicable.

Signed-off-by: Fernando Ramos <greenfoo@u92.eu>
Reviewed-by: Sean Paul <sean@poorly.run>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210924064324.229457-18-greenfoo@u92.eu