amdgpu: fix a few kernel-doc markup issues

A kernel-doc markup can't be mixed with a random comment,
as it causes parsing problems.

While here, change an invalid kernel-doc markup into
a common comment.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/e899f50404e94ac9a7c3267dd34f951c1a44fb2b.1603791716.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>