drm/amdgpu: switch to SMN interface to operate RSMU index mode

This makes consistent with other regsiters' access in this module.

Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Tao Zhou <tao.zhou1@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>