drm/amd/pm: create a new holder for those APIs used only by legacy ASICs(si/kv)

Those APIs are used only by legacy ASICs(si/kv). They cannot be
shared by other ASICs. So, we create a new holder for them.

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>