drm/amdgpu: declare smuio v13_0 callbacks as static

fix -Wmissing-protoypes warning

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Likun Gao <Likun.Gao@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>