drm/todo: Add s/dev_*/DRM_DEV_*/ coversion to TODO

Now that we have the DRM_DEV_* variants, we should use them.

Signed-off-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170908143218.9701-1-seanpaul@chromium.org