drm/amdgpu: refactor function to init no-psp fw

Refactor the code of amdgpu_ucode_init_single_fw to make it more
readable as too many ucode need to handle on this function currently.

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>