drm/amd/pm: do not expose the API used internally only in kv_dpm.c

Move it to kv_dpm.c instead.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>