drm/amd/amdgpu: fix Tonga S3 resume hang on rhel6.8

1. security firmware loading has moved to sw init, so this code
   is useless.
2. it seems that driver could not call request_firmware on
   kernel 2.6, when S3 resume. for request firmware depends on
   userspace, at this time, userspace is freeze.

Signed-off-by: Jim Qu <Jim.Qu@amd.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>