drm/amdgpu/umsch: fix psp frontdoor loading

These changes are missed in rebase.

Signed-off-by: Lang Yu <Lang.Yu@amd.com>
Reviewed-by: Veerabadhran Gopalakrishnan <Veerabadhran.Gopalakrishnan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>