Documentation: Fix typo error in todo.rst

Change wrong function name drm_modest_lock_all() to drm_modeset_lock_all()

Signed-off-by: Sidong Yang <realwakka@gmail.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200528172159.24641-1-realwakka@gmail.com