drm/amdgpu/psp: Fix can't detect psp INVOKE command failed

There isn't ucode when executing INVOKE command, so current code can't
check the failure of INVOKE command.

Remove the ucode check.

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>