drm/amdgpu/psp: avoid hard-code fence value pre submission

Hard-code submission fence is not a sustainable way as there is
more and more run-time psp kernel mode submission from driver to
fw

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>