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.

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>