docs: drm: remove no longer relevant TODO entry

This entry asked to rename all drm core "*_reference/_unrefence"
functions to "*_get/_put".

Now that this task is complete, we can remove this entry from the TODO
list.

Signed-off-by: Fernando Ramos <greenfoo@gluegarage.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20181115221634.22715-10-greenfoo@gluegarage.com