drm: correct comments

On failure, these functions return error pointer, not NULL.

Signed-off-by: Liu Zixian <liuzixian4@huawei.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220718015357.1722-1-liuzixian4@huawei.com