drm/amdgpu/psp: Fix to get wrong xgmi session id

Driver get session id after loading TA FW and the session id is used
by driver instances to communicate with TA. PF and VF have different
session id.

xGMI session id should get from response buffer, correct it.

Signed-off-by: Xiangliang Yu <Xiangliang.Yu@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>