drm/amdgpu:Refine handshake of mailbox

Signed-off-by: Ken Xue <Ken.Xue@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Xiangliang Yu <Xiangliang.Yu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>