drm/amdgpu/si/dpm: make a bunch of things static

remnants of the port from radeon.

Acked-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>