drm/amdgpu: add helper function to init sos ucode

driver already had psp_firmware_header struture to
deal with different layout of sos ucode. the sos
micorcode initialization could be common one.

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