drm/amd/pm: add some basic functions to support umd P-state function for vangogh.

This patch is to add some basic functions to support
umd P-state function for vangogh.

Signed-off-by: Xiaojian Du <Xiaojian.Du@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>