drm/amdgpu: Add SRIOV mailbox backend for Navi1x

Mimic the ones for Vega10, add mailbox backend for Navi1x

Reviewed-by: Emily Deng <Emily.Deng@amd.com>
Signed-off-by: Jiange Zhao <Jiange.Zhao@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>