drm/amdgpu: set fw load type for navy_flounder

Currently navy_flounder only supports backdoor loading type.
Will switch to psp load type when psp is ready.

Signed-off-by: Jiansong Chen <Jiansong.Chen@amd.com>
Reviewed-by: Tao Zhou <tao.zhou1@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>