drm/displayid: return struct displayid_header from validate_displayid()

Avoid figuring out the header pointer multiple times.

Cc: Iaroslav Boliukin <iam@lach.pw>
Cc: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/e840c744d165608a41db5a5e2bd4b0205783c697.1676580180.git.jani.nikula@intel.com