drm/amdgpu/soc15: use atomfirmware for setting bios scratch for reset

Need to use the atomfirmware interface rather than atombios since
soc15 is atomfirmware based.

Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>