drm/amd: Remove errors from sphinx documentation

Eliminating the warnings produced by sphinx when processing the sphinx comments in
 amdgpu_device.c & amdgpu_mn.c

Signed-off-by: Darren Powell <darren.powell@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>