drm/amdgpu: fix documentation of amdgpu_mn.c v2

And wire it up as well.

v2: improve the wording, fix label mismatch

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>