drm/doc: Update todo.rst

We're using this for outreachy, unfortunately someone already tried to
look at a task that was done already :-(

Update them all.

Cc: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20170911065151.22672-1-daniel.vetter@ffwll.ch