drm/amdgpu/psp: switch to use sos_offset_bytes member as sys_bin_size

Navi10 will have toc built-in sos binary so that using header.ucode_size_bytes
minus sos_size_bytes actually is not sys_bin_size.

Using sos_offset_bytes works for both vega20 (psp_firmware_header_v1_0) and
navi10 (psp_firmware_header_v1_1)

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