drm_dp_cec.c: fix formatting typo: %pdH -> %phD

This caused a kernel oops since %pdH interpreted the pointer
as a struct file.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/f3720ddf-ec0f-cd22-46b6-720a5e2098f2@xs4all.nl