drm/amdgpu: fix amdgpu_bo_release_notify() comment error

fix amdgpu_bo_release_notify() comment error.

Signed-off-by: Kevin Wang <kevin1.wang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Dennis Li <Dennis.Li@amd.com>
Acked-by: Nirmoy Das <nirmoy.das@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>