drm/amdgpu: add interface amdgpu_gfx_init_spm_golden for Navi1x

On Navi1x, the SPM golden settings are lost after GFXOFF
enter/exit, so reconfiguration is needed. Make the
configuration code as an interface for future use.

Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Signed-off-by: Tianci.Yin <tianci.yin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>