drm/amdgpu: correct psp ucode arrary start address

For ASICs that need to load sys_drv_aux and sos_aux,
the sys_start_addr is not the start address of psp
ucode array because the sys_drv_aux and sos_aux actaully
located at the end of the ucode array, instead, the
psp ucode arrary start address should be sos_hdr +
sos_hdr_offset.

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